dt-bindings: arm: imx: Add the soc binding for i.MX8MN

This patch adds the soc & board binding for i.MX8MN.
Signed-off-by: NAnson Huang <Anson.Huang@nxp.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -215,6 +215,12 @@ properties:
               - fsl,imx8mm-evk            # i.MX8MM EVK Board
           - const: fsl,imx8mm
 
+      - description: i.MX8MN based Boards
+        items:
+          - enum:
+              - fsl,imx8mn-ddr4-evk       # i.MX8MN DDR4 EVK Board
+          - const: fsl,imx8mn
+
       - description: i.MX8MQ based Boards
         items:
           - enum: