bindings: arm: fsl: Add PHYTEC i.MX8MM devicetree bindings

Add devicetree bindings for i.MX8MM based phyCORE-i.MX8MM
and phyBOARD-Polis RDK.
Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -872,6 +872,12 @@ properties:
           - const: toradex,verdin-imx8mm          # Verdin iMX8M Mini Module
           - const: fsl,imx8mm
 
+      - description: PHYTEC phyCORE-i.MX8MM SoM based boards
+        items:
+          - const: phytec,imx8mm-phyboard-polis-rdk # phyBOARD-Polis RDK
+          - const: phytec,imx8mm-phycore-som        # phyCORE-i.MX8MM SoM
+          - const: fsl,imx8mm
+
       - description: Variscite VAR-SOM-MX8MM based boards
         items:
           - const: variscite,var-som-mx8mm-symphony