drm/bridge: Add sii902x DT bindings doc

sii902x HDMI bridge bindings

Required properties:

	- compatible: "sil,sii9022"

	- reg: i2c address of the bridge

Optional properties:

	- interrupts-extended or interrupt-parent + interrupts: describe

	  the interrupt line used to inform the host about hotplug events.

	- reset-gpios: OF device-tree gpio specification for RST_N pin.

Optional subnodes:

	- video input: this subnode can contain a video input port node

	  to connect the bridge to a display controller output (See this

	  documentation [1]).

[1]: Documentation/devicetree/bindings/media/video-interfaces.txt

Example:

	hdmi-bridge@39 {

		compatible = "sil,sii9022";

		reg = <0x39>;

		reset-gpios = <&pioA 1 0>;

		ports {

			#address-cells = <1>;

			#size-cells = <0>;

			port@0 {

				reg = <0>;

				bridge_in: endpoint {

					remote-endpoint = <&dc_out>;

				};

			};

		};

	};