clocksource: sun4i: Add clocksource and sched clock drivers

Use the second timer found on the Allwinner SoCs as a clock source and
sched clock, that were both not used yet on these platforms.

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