dt-bindings: atmel, at91rm9200-rtc: add microchip, sam9x60-rtc

Add microchip,sam9x60-rtc to DT bindings documentation.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1579085987-13976-2-git-send-email-claudiu.beznea@microchip.com


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