dt-bindings: arm: imx: Add the soc binding for imx8mm

Add the soc & board binding for i.MX8MM.
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -155,6 +155,12 @@ properties:
           - const: compulab,cl-som-imx7
           - const: fsl,imx7d
 
+      - description: i.MX8MM based Boards
+        items:
+          - enum:
+              - fsl,imx8mm-evk            # i.MX8MM EVK Board
+          - const: fsl,imx8mm
+
       - description: i.MX8QXP based Boards
         items:
           - enum: