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>;
        };