drm/bridge: Add sii902x DT bindings doc

Add Sii9022 DT bindings description.
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NRob Herring <robh@kernel.org>
---
Changes since v6:
- make 'reset-gpios' optional

Changes since v1:
- rename doc file
- s/sil902/sii902/

Documentation/devicetree/bindings/display/bridge/sii902x.txt

+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>;
+				};
+			};
+		};
+	};