ARM: sun4i/sun7i: RTC driver

This patch introduces the driver for the RTC in the Allwinner A10 and
A20 SoCs.

Signed-off-by: Carlo Caione <carlo.caione@gmail.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>