dt-bindings: rtc: Convert Allwinner A31 RTC to a schema

The newer Allwinner SoCs have an embedded RTC supported in Linux, with a
matching Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>