Documentation: bindings: add dt documentation for Rockchip eMMC PHY

Rockchip EMMC PHY

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Required properties:

 - compatible: rockchip,rk3399-emmc-phy

 - rockchip,grf : phandle to the syscon managing the "general

   register files"

 - #phy-cells: must be 0

 - reg: PHY configure reg address offset in "general

   register files"

Example:

emmcphy: phy {

	compatible = "rockchip,rk3399-emmc-phy";

	rockchip,grf = <&grf>;

	reg = <0xf780>;

	#phy-cells = <0>;

};