dt-bindings: phy: add binding for Freescale i.MX8MQ USB3 PHY

* Freescale i.MX8MQ USB3 PHY binding

Required properties:

- compatible:	Should be "fsl,imx8mq-usb-phy"

- #phys-cells:	must be 0 (see phy-bindings.txt in this directory)

- reg:		The base address and length of the registers

- clocks:	phandles to the clocks for each clock listed in clock-names

- clock-names:	must contain "phy"

Example:

	usb3_phy0: phy@381f0040 {

		compatible = "fsl,imx8mq-usb-phy";

		reg = <0x381f0040 0x40>;

		clocks = <&clk IMX8MQ_CLK_USB1_PHY_ROOT>;

		clock-names = "phy";

		#phy-cells = <0>;

	};