The current code to initialize, register and read the clocksource is
already factored out in mmio.c via the clocksource_mmio_init function.

The only difference is the readl vs readl_relaxed.

Factor out the code with the clocksource_mmio_init function.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>