dt-bindings: rtc: add bindings for MT2712 RTC

Document the binding for MT2712 RTC implemented by rtc-mt2712.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ran Bi <ran.bi@mediatek.com>
Link: https://lore.kernel.org/r/20200226051303.22560-2-ran.bi@mediatek.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>