dt-bindings: add document for dw_hdmi

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/drm/bridge/dw_hdmi.txt

+DesignWare HDMI bridge bindings
+
+Required properties:
+- compatible: platform specific such as:
+   * "snps,dw-hdmi-tx"
+   * "fsl,imx6q-hdmi"
+   * "fsl,imx6dl-hdmi"
+   * "rockchip,rk3288-dw-hdmi"
+- reg: Physical base address and length of the controller's registers.
+- interrupts: The HDMI interrupt number
+- clocks, clock-names : must have the phandles to the HDMI iahb and isfr clocks,
+  as described in Documentation/devicetree/bindings/clock/clock-bindings.txt,
+  the clocks are soc specific, the clock-names should be "iahb", "isfr"
+-port@[X]: SoC specific port nodes with endpoint definitions as defined
+   in Documentation/devicetree/bindings/media/video-interfaces.txt,
+   please refer to the SoC specific binding document:
+    * Documentation/devicetree/bindings/drm/imx/hdmi.txt
+    * Documentation/devicetree/bindings/video/dw_hdmi-rockchip.txt
+
+Optional properties
+- reg-io-width: the width of the reg:1,4, default set to 1 if not present
+- ddc-i2c-bus: phandle of an I2C controller used for DDC EDID probing
+- clocks, clock-names: phandle to the HDMI CEC clock, name should be "cec"
+
+Example:
+	hdmi: hdmi@0120000 {
+		compatible = "fsl,imx6q-hdmi";
+		reg = <0x00120000 0x9000>;
+		interrupts = <0 115 0x04>;
+		gpr = <&gpr>;
+		clocks = <&clks 123>, <&clks 124>;
+		clock-names = "iahb", "isfr";
+		ddc-i2c-bus = <&i2c2>;
+
+		port@0 {
+			reg = <0>;
+
+			hdmi_mux_0: endpoint {
+				remote-endpoint = <&ipu1_di0_hdmi>;
+			};
+		};
+
+		port@1 {
+			reg = <1>;
+
+			hdmi_mux_1: endpoint {
+				remote-endpoint = <&ipu1_di1_hdmi>;
+			};
+		};
+	};