dt-bindings: add document for rockchip dp phy

Rockchip specific extensions to the Analogix Display Port PHY

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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>;

};