dt-bindings: ARM: at91: document exegin q5xr5 board

Document Exegin Q5xR5 board.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210816064416.1630674-7-claudiu.beznea@microchip.com

Documentation/devicetree/bindings/arm/atmel-at91.yaml

@@ -132,6 +132,12 @@ properties:
           - const: atmel,at91sam9g20
           - const: atmel,at91sam9
 
+      - description: Exegin Q5xR5 board
+        items:
+          - const: exegin,q5xr5
+          - const: atmel,at91sam9g20
+          - const: atmel,at91sam9
+
       - items:
           - enum:
               - atmel,sama5d31