timer: sti: use clk API to get timer clock rate

Retrieve clock rate through device tree. This mimics the behavior of
arm_global_timer in Linux.

Signed-off-by: Nicolas Heemeryck <nicolas.heemeryck@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
33 jobs for u-boot-stm32-20200324 in 95 minutes and 14 seconds (queued for 75 minutes and 37 seconds)
latest
Status Job ID Name Coverage
  Testsuites
passed #68446
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Build envtools

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passed #68445
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Build tools-only

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passed #68444
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Check for configs without MAINTAINERS entry

00:00:22

passed #68447
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Run binman, buildman, dtoc, Kconfig and patman testsuites

00:01:07

passed #68440
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cppcheck

00:06:45

passed #68441
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grep TODO/FIXME/HACK

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passed #68442
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htmldocs

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passed #68443
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sloccount

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  Test.Py
passed #68451
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evb-ast2500 test.py

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passed #68455
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integratorcp_cm926ejs test.py

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passed #68462
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qemu-ppce500 test.py

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passed #68463
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qemu-riscv64 test.py

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passed #68464
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qemu-x86 test.py

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passed #68465
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qemu-x86_64 test.py

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passed #68456
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qemu_arm test.py

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passed #68457
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qemu_arm64 test.py

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passed #68458
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qemu_mips test.py

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passed #68460
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qemu_mips64 test.py

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passed #68461
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qemu_mips64el test.py

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qemu_mipsel test.py

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passed #68448
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sandbox test.py

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passed #68449
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sandbox with clang test.py

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passed #68452
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sandbox_flattree test.py

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passed #68450
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sandbox_spl test.py

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passed #68453
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vexpress_ca15_tc2 test.py

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passed #68454
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vexpress_ca9x4 test.py

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passed #68467
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xilinx_versal_virt test.py

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passed #68468
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xtfpga test.py

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passed #68466
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zynq_zc702 test.py

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  World Build
passed #68469
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build all 32bit ARM platforms

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passed #68470
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build all 64bit ARM platforms

00:33:55

passed #68471
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build all PowerPC platforms

00:35:31

passed #68472
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build all other platforms

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