dt-bindings: rtc: at91rm9200: add sam9x7 compatible

Add compatible for SAM9X7 RTC.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230728102710.266457-1-varshini.rajendran@microchip.com


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