clocksource: sun4i: Select CLKSRC_MMIO

The Allwinner SoCs timer use the clocksource MMIO functions. We thus
need to select them in Kconfig.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>