dt-bindings: serial: atmel,at91-usart: update compatible for sam9x60

There is only one debug unit in the sam9x60 SOC and it has the chipid
register. So, the dbgu compatible strings are valid only for debug usart.
Defining these dbgu compatible strings are not valid for flexcom usart.
So adding the items which is valid only for flexcom usart and removing
the microchip,sam9x60-usart compatible string from the enum list as no
usart node defines only this specific compatible string.

Signed-off-by: Durai Manickam KR <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnd Bergmann <[e-mail]>

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

···
       - enum:
           - atmel,at91rm9200-usart
           - atmel,at91sam9260-usart
-          - microchip,sam9x60-usart
       - items:
           - const: atmel,at91rm9200-dbgu
           - const: atmel,at91rm9200-usart
       - items:
           - const: atmel,at91sam9260-dbgu
+          - const: atmel,at91sam9260-usart
+      - items:
+          - const: microchip,sam9x60-usart
           - const: atmel,at91sam9260-usart
       - items:
           - const: microchip,sam9x60-dbgu