dt-bindings: serial: atmel,at91-usart: add microchip,sama7d65-usart

Add SAMA7D65 USART compatible to DT bindings documentation.

Signed-off-by: Dharma Balasubiramani <[e-mail]>
Signed-off-by: Ryan Wanner <[e-mail]>
Acked-by: "Rob Herring (Arm)" <[e-mail]>
Link: https://lore.kernel.org/r/ba1[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml

···
           - enum:
               - microchip,sam9x60-usart
               - microchip,sam9x7-usart
+              - microchip,sama7d65-usart
           - const: atmel,at91sam9260-usart
       - items:
           - const: microchip,sam9x60-dbgu