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>