Documentation: bindings: add dt documentation for cdn DP controller

This patch adds a binding that describes the cdn DP controller for
rk3399.

Signed-off-by: Chris Zhong <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Enric Balletbo i Serra <[e-mail]>
[document the missing 3 reset-names]
Signed-off-by: Heiko Stuebner <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/rockchip/cdn-dp-rockchip.txt

···
+Rockchip RK3399 specific extensions to the cdn Display Port
+================================
+
+Required properties:
+- compatible: must be "rockchip,rk3399-cdn-dp"
+
+- reg: physical base address of the controller and length
+
+- clocks: from common clock binding: handle to dp clock.
+
+- clock-names: from common clock binding:
+	       Required elements: "core-clk" "pclk" "spdif" "grf"
+
+- resets : a list of phandle + reset specifier pairs
+- reset-names : string of reset names
+		Required elements: "apb", "core", "dptx", "spdif"
+- power-domains : power-domain property defined with a phandle
+		  to respective power domain.
+- assigned-clocks: main clock, should be <&cru SCLK_DP_CORE>
+- assigned-clock-rates : the DP core clk frequency, shall be: 100000000
+
+- rockchip,grf: this soc should set GRF regs, so need get grf here.
+
+- ports: contain a port nodes with endpoint definitions as defined in
+	 Documentation/devicetree/bindings/media/video-interfaces.txt.
+	 contained 2 endpoints, connecting to the output of vop.
+
+- phys: from general PHY binding: the phandle for the PHY device.
+
+- extcon: extcon specifier for the Power Delivery
+
+- #sound-dai-cells = it must be 1 if your system is using 2 DAIs: I2S, SPDIF
+
+-------------------------------------------------------------------------------
+
+Example:
+	cdn_dp: dp@fec00000 {
+		compatible = "rockchip,rk3399-cdn-dp";
+		reg = <0x0 0xfec00000 0x0 0x100000>;
+		interrupts = <GIC_SPI 9 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&cru SCLK_DP_CORE>, <&cru PCLK_DP_CTRL>,
+			 <&cru SCLK_SPDIF_REC_DPTX>, <&cru PCLK_VIO_GRF>;
+		clock-names = "core-clk", "pclk", "spdif", "grf";
+		assigned-clocks = <&cru SCLK_DP_CORE>;
+		assigned-clock-rates = <100000000>;
+		power-domains = <&power RK3399_PD_HDCP>;
+		phys = <&tcphy0_dp>, <&tcphy1_dp>;
+		resets = <&cru SRST_DPTX_SPDIF_REC>;
+		reset-names = "spdif";
+		extcon = <&fusb0>, <&fusb1>;
+		rockchip,grf = <&grf>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		#sound-dai-cells = <1>;
+
+		ports {
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			dp_in: port {
+				#address-cells = <1>;
+				#size-cells = <0>;
+				dp_in_vopb: endpoint@0 {
+					reg = <0>;
+					remote-endpoint = <&vopb_out_dp>;
+				};
+
+				dp_in_vopl: endpoint@1 {
+					reg = <1>;
+					remote-endpoint = <&vopl_out_dp>;
+				};
+			};
+		};
+	};