dt-bindings: at91rm9260-rtt: add sam9x7 compatible

Add compatible for SAM9X7 RTT.
Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240223172552.672094-1-varshini.rajendran@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/atmel,at91sam9260-rtt.yaml

@@ -19,7 +19,9 @@ properties:
       - items:
           - const: atmel,at91sam9260-rtt
       - items:
-          - const: microchip,sam9x60-rtt
+          - enum:
+              - microchip,sam9x60-rtt
+              - microchip,sam9x7-rtt
           - const: atmel,at91sam9260-rtt
       - items:
           - const: microchip,sama7g5-rtt