dt-bindings: net: fsl,qoriq-mc-dpmac: add missed property phys

Add missed property phys, which indicate how connect to serdes phy.
Fix below warning:
arch/arm64/boot/dts/freescale/fsl-lx2160a-honeycomb.dtb: fsl-mc@80c000000: dpmacs:ethernet@7: Unevaluated properties are not allowed ('phys' was unexpected)
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl,qoriq-mc-dpmac.yaml

@@ -38,6 +38,10 @@ properties:
 
   managed: true
 
+  phys:
+    description: A reference to the SerDes lane(s)
+    maxItems: 1
+
 required:
   - reg