dt-bindings: rtc: Add Mstar MSC313e RTC devicetree bindings documentation

This adds the documentation for the devicetree bindings of the Mstar
MSC313e RTC driver, found from MSC313e SoCs and newer.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210823171613.18941-2-romain.perier@gmail.com