dt-bindings: arm: at91: Document Kizboxmini and Smartkiz boards binding

Document devicetree's bindings for the Overkiz's Kizbox Mini and
Smartkiz boards, based on a SAM9G25 Atmel SoC.
Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191220103835.160154-1-kamel.bouhara@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -35,6 +35,16 @@ properties:
               - atmel,at91sam9x60
           - const: atmel,at91sam9
 
+      - items:
+          - enum:
+              - overkiz,kizboxmini-base # Overkiz kizbox Mini Base Board
+              - overkiz,kizboxmini-mb   # Overkiz kizbox Mini Mother Board
+              - overkiz,kizboxmini-rd   # Overkiz kizbox Mini RailDIN
+              - overkiz,smartkiz        # Overkiz SmartKiz Board
+          - const: atmel,at91sam9g25
+          - const: atmel,at91sam9x5
+          - const: atmel,at91sam9
+
       - items:
           - enum:
               - atmel,at91sam9g15