rtc: sun6i: Add sun6i RTC driver

This patch introduces the driver for the RTC in the Allwinner A31 and
A23 SoCs.

Unlike the RTC found in A10/A20 SoCs, which was part of the timer, the
RTC in A31/A23 are a separate hardware block, which also contain a few
controls for the RTC block hardware (a regulator and RTC block GPIO pin
latches), while also having separate interrupts for the alarms.

The hardware is different enough to make a different driver for it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
rtc: sunxi: Depend on platforms sun4i/sun7i that actually have the rtc

Now that we have Kconfig options for individual sunxi platforms, let
the rtc-sunxi driver depend on the platforms that actually have this
hardware, sun4i and sun7i.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
ARM: dts: sun6i: add rtc device node

Now that we have a driver for sun6i's rtc hardware, add a device node
for it so we can use it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
ARM: dts: sun8i: add rtc device node

sun8i shares the same rtc hardware as sun6i. Now that we have a driver
for it, add a device node to the DTSI for it so we can use it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
ARM: sunxi: Add A31 RTC driver to sunxi_defconfig

Now that we have a driver for A31's RTC, enable it
in the default sunxi config.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>