dt-bindings: phy: Add documentation for mixel dphy

Mixel DSI PHY for i.MX8

The Mixel MIPI-DSI PHY IP block is e.g. found on i.MX8 platforms (along the

MIPI-DSI IP from Northwest Logic). It represents the physical layer for the

electrical signals for DSI.

Required properties:

- compatible: Must be:

  - "fsl,imx8mq-mipi-dphy"

- clocks: Must contain an entry for each entry in clock-names.

- clock-names: Must contain the following entries:

  - "phy_ref": phandle and specifier referring to the DPHY ref clock

- reg: the register range of the PHY controller

- #phy-cells: number of cells in PHY, as defined in

  Documentation/devicetree/bindings/phy/phy-bindings.txt

  this must be <0>

Optional properties:

- power-domains: phandle to power domain

Example:

	dphy: dphy@30a0030 {

		compatible = "fsl,imx8mq-mipi-dphy";

		clocks = <&clk IMX8MQ_CLK_DSI_PHY_REF>;

		clock-names = "phy_ref";

		reg = <0x30a00300 0x100>;

		power-domains = <&pd_mipi0>;

		#phy-cells = <0>;

        };