dt-bindings: rtc: convert at91sam9 bindings to json-schema

Convert RTC binding for Atmel/Microchip SoCs to Device Tree Schema
format.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220304161159.147784-4-sergiu.moga@microchip.com