clocksource/drivers/sun5i: Add clock notifiers

clocksource/drivers/sun5i: Add clock notifiers

The parent clock of the sun5i timer is the AHB clock, which rate
might change because of other devices requirements.

This is for example the case on the Allwinner A31, where the DMA
controller needs a minimum rate higher than the default, that is
enforced after the timer driver has probed.

Add clock notifiers to make sure we reflect the clock rate
changes in the timer rates.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1427796746-373-5-git-send-email-daniel.lezcano@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>