Documentation: bridge: Add documentation for ps8622 DT properties

Add documentation for DT properties supported by ps8622/ps8625
eDP-LVDS converter.
Signed-off-by: NAjay Kumar <ajaykumar.rs@samsung.com>
Acked-by: NInki Dae <inki.dae@samsung.com>
Tested-by: NRahul Sharma <rahul.sharma@samsung.com>
Tested-by: NJavier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>
Tested-by: NSjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: NThierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/video/bridge/ps8622.txt

+ps8622-bridge bindings
+
+Required properties:
+	- compatible: "parade,ps8622" or "parade,ps8625"
+	- reg: first i2c address of the bridge
+	- sleep-gpios: OF device-tree gpio specification for PD_ pin.
+	- reset-gpios: OF device-tree gpio specification for RST_ pin.
+
+Optional properties:
+	- lane-count: number of DP lanes to use
+	- use-external-pwm: backlight will be controlled by an external PWM
+	- video interfaces: Device node can contain video interface port
+			    nodes for panel according to [1].
+
+[1]: Documentation/devicetree/bindings/media/video-interfaces.txt
+
+Example:
+	lvds-bridge@48 {
+		compatible = "parade,ps8622";
+		reg = <0x48>;
+		sleep-gpios = <&gpc3 6 1 0 0>;
+		reset-gpios = <&gpc3 1 1 0 0>;
+		lane-count = <1>;
+		ports {
+			port@0 {
+				bridge_out: endpoint {
+					remote-endpoint = <&panel_in>;
+				};
+			};
+		};
+	};