ARM: dts: sun4i/sun7i: add RTC node

Add the RTC node to DTS for Allwinner A10 and Allwinner A20.

Signed-off-by: Carlo Caione <carlo.caione@gmail.com>
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v1->v2:
- changed string "compatible" in sun4i-a10.dtsi
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>
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v1->v2:
- Patch split in two
- Fix coding style
- New driver description
- Rework macros
- Add comments
- Change while in for cycles