Add CLK support to enable AHB and MOD SPI clocks on sun4i_spi driver.

Clock disablement could be done while releasing the bus transfer, but
the existing code doesn't disable the clocks it only taken care of clock
enablement globally in probe.

So to make a proper clock handling, the clocks should enable it in claim
and disable it in release.

This patch would also do that change, by enable and disable clock in
proper order.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>