dt-bindings: add document for rockchip dp phy

Add dt binding documentation for rockchip display port PHY.
Signed-off-by: NYakir Yang <ykk@rock-chips.com>
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NHeiko Stuebner <heiko@sntech.de>
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/rockchip-dp-phy.txt

+Rockchip specific extensions to the Analogix Display Port PHY
+------------------------------------
+
+Required properties:
+- compatible : should be one of the following supported values:
+	 - "rockchip.rk3288-dp-phy"
+- clocks: from common clock binding: handle to dp clock.
+	of memory mapped region.
+- clock-names: from common clock binding:
+	Required elements: "24m"
+- rockchip,grf: phandle to the syscon managing the "general register files"
+- #phy-cells : from the generic PHY bindings, must be 0;
+
+Example:
+
+edp_phy: edp-phy {
+	compatible = "rockchip,rk3288-dp-phy";
+	rockchip,grf = <&grf>;
+	clocks = <&cru SCLK_EDP_24M>;
+	clock-names = "24m";
+	#phy-cells = <0>;
+};