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

Documentation/devicetree/bindings/rtc/atmel,at91rm9200-rtc.txt

 Atmel AT91RM9200 Real Time Clock
 
 Required properties:
-- compatible: should be: "atmel,at91rm9200-rtc" or "atmel,at91sam9x5-rtc"
+- compatible: should be: "atmel,at91rm9200-rtc", "atmel,at91sam9x5-rtc" or
+  "microchip,sam9x60-rtc"
 - reg: physical base address of the controller and length of memory mapped
   region.
 - interrupts: rtc alarm/event interrupt