dt-bindings: arm: fsl: add RVE gateway board

Add DT compatible string for RVE gateway board based on a Variscite
VAR-SOM-NANO with a NXP MX8MN nano CPU.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -1022,7 +1022,9 @@ properties:
 
       - description: Variscite VAR-SOM-MX8MN based boards
         items:
-          - const: variscite,var-som-mx8mn-symphony
+          - enum:
+              - rve,rve-gateway # i.MX8MN RVE Gateway Board
+              - variscite,var-som-mx8mn-symphony
           - const: variscite,var-som-mx8mn
           - const: fsl,imx8mn