dt-bindings: arm: at91: Document Kizbox2-2 board binding

Document devicetree's binding for the Kizbox2-2 board of
Overkiz SAS based on SAMA5D31 SoC.
Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191105212234.22999-1-kamel.bouhara@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -80,6 +80,13 @@ properties:
           - const: atmel,sama5d3
           - const: atmel,sama5
 
+      - description: Overkiz kizbox2 board with two heads
+        items:
+          - const: overkiz,kizbox2-2
+          - const: atmel,sama5d31
+          - const: atmel,sama5d3
+          - const: atmel,sama5
+
       - items:
           - enum:
               - atmel,sama5d31