video: Add DT binding documentation for VGA connector

The VGA connector is described by a single input port and an optional
DDC bus.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

Documentation/devicetree/bindings/video/vga-connector.txt

+VGA Connector
+=============
+
+Required properties:
+
+- compatible: "vga-connector"
+
+Optional properties:
+
+- label: a symbolic name for the connector corresponding to a hardware label
+- ddc-i2c-bus: phandle to the I2C bus that is connected to VGA DDC
+
+Required nodes:
+
+The VGA connector internal connections are modeled using the OF graph bindings
+specified in Documentation/devicetree/bindings/graph.txt.
+
+The VGA connector has a single port that must be connected to a video source
+port.
+
+
+Example
+-------
+
+vga0: connector@0 {
+	compatible = "vga-connector";
+	label = "vga";
+
+	ddc-i2c-bus = <&i2c3>;
+
+	port {
+		vga_connector_in: endpoint {
+			remote-endpoint = <&adv7123_out>;
+		};
+	};
+};