Accessing the registers of the RTC block on Tegra requires the module
clock to be enabled. This only works because the RTC module clock will
be enabled by default during early boot. However, because the clock is
unused, the CCF will disable it at late_init time. This causes the RTC
to become unusable afterwards. This can easily be reproduced by trying
to use the RTC:

	$ hwclock --rtc /dev/rtc1

This will hang the system. I ran into this by following up on a report
by Martin Michlmayr that reboot wasn't working on Tegra210 systems. It
turns out that the rtc-tegra driver's ->shutdown() implementation will
hang the CPU, because of the disabled clock, before the system can be
rebooted.

What confused me for a while is that the same driver is used on prior
Tegra generations where the hang can not be observed. However, as Peter
De Schrijver pointed out, this is because on 32-bit Tegra chips the RTC
clock is enabled by the tegra20_timer.c clocksource driver, which uses
the RTC to provide a persistent clock. This code is never enabled on
64-bit Tegra because the persistent clock infrastructure does not exist
on 64-bit ARM.

The proper fix for this is to add proper clock handling to the RTC
driver in order to ensure that the clock is enabled when the driver
requires it. All device trees contain the clock already, therefore
no additional changes are required.

Reported-by: Martin Michlmayr <tbm@cyrius.com>
Acked-By Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>