dt-bindings: arm: add sam9x60-ek board

Add documentation for SAM9X60-EK board.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-16-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -82,6 +82,12 @@ properties:
           - const: atmel,sama5d2
           - const: atmel,sama5
 
+      - description: SAM9X60-EK board
+        items:
+          - const: microchip,sam9x60ek
+          - const: microchip,sam9x60
+          - const: atmel,at91sam9
+
       - description: Nattis v2 board with Natte v2 power board
         items:
           - const: axentia,nattis-2