dt-bindings: arm: Add DH electronics i.MX8M Plus DHCOM on PDK3

Add DT compatible string for DH electronics i.MX8M Plus DHCOM on PDK3
evaluation board into YAML DT binding document. This setup is a general
purpose SoM evaluation board.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: Marek Vasut

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -1025,7 +1025,9 @@ properties:
 
       - description: i.MX8MP DHCOM based Boards
         items:
-          - const: dh,imx8mp-dhcom-pdk2      # i.MX8MP DHCOM SoM on PDK2 board
+          - enum:
+              - dh,imx8mp-dhcom-pdk2         # i.MX8MP DHCOM SoM on PDK2 board
+              - dh,imx8mp-dhcom-pdk3         # i.MX8MP DHCOM SoM on PDK3 board
           - const: dh,imx8mp-dhcom-som       # i.MX8MP DHCOM SoM
           - const: fsl,imx8mp