video: Add ADV7123 DT bindings documentation

The ADV7123 is a video DAC described by an input port, an output port,
and an optional power save GPIO.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

Documentation/devicetree/bindings/video/adi,adv7123.txt

+Analog Device ADV7123 Video DAC
+-------------------------------
+
+The ADV7123 is a digital-to-analog converter that outputs VGA signals from a
+parallel video input.
+
+Required properties:
+
+- compatible: Should be "adi,adv7123"
+
+Optional properties:
+
+- psave-gpios: Power save control GPIO
+
+Required nodes:
+
+The ADV7123 has two video ports. Their connections are modeled using the OF
+graph bindings specified in Documentation/devicetree/bindings/graph.txt.
+
+- Video port 0 for DPI input
+- Video port 1 for VGA output
+
+
+Example
+-------
+
+	adv7123: encoder@0 {
+		compatible = "adi,adv7123";
+
+		ports {
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			port@0 {
+				reg = <0>;
+
+				adv7123_in: endpoint@0 {
+					remote-endpoint = <&dpi_out>;
+				};
+			};
+
+			port@1 {
+				reg = <1>;
+
+				adv7123_out: endpoint@0 {
+					remote-endpoint = <&vga_connector_in>;
+				};
+			};
+		};
+	};