Device-Tree bindings for hisilicon DSI controller driver

A DSI controller resides in the middle of display controller and external
HDMI converter.

Required properties:
- compatible: value should be one of the following
	"hisilicon,hi6220-dsi".
- reg: physical base address and length of the controller's registers.
- clocks: the clocks needed.
- clock-names: the name of the clocks.
- port: DSI controller output port. This contains one endpoint subnode, with its
  remote-endpoint set to the phandle of the connected external HDMI endpoint.
  See Documentation/devicetree/bindings/graph.txt for device graph info.

A example of HiKey board hi6220 SoC and board specific DT entry:
Example:

SoC specific:
	dsi: dsi@0xf4107800 {
		compatible = "hisilicon,hi6220-dsi";
		reg = <0x0 0xf4107800 0x0 0x100>;
		clocks = <&media_ctrl  HI6220_DSI_PCLK>;
		clock-names = "pclk_dsi";

		port {
			dsi_out: endpoint {
				remote-endpoint = <&adv_in>;
			};
		};

	};

Board specific:
	i2c2: i2c@f7102000 {
		status = "ok";

		adv7533: adv7533@39 {
			compatible = "adi,adv7533";
			reg = <0x39>;
			interrupt-parent = <&gpio1>;
			interrupts = <1 2>;
			pd-gpio = <&gpio0 4 0>;
			adi,dsi-lanes = <4>;

			port {
				adv_in: endpoint {
					remote-endpoint = <&dsi_out>;
				};
			};
		};
	};

