dt-bindings: rtc: at91rm9200: convert bindings to json-schema

Convert Real Time Clock for Atmel/Microchip SoCs bindings documentation
to json-schema.

Cc: Rob Herring <robh+dt@kernel.org>
Link: https://lore.kernel.org/r/20191229204421.337612-2-alexandre.belloni@bootlin.com


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