Documentation: bindings: add dt documentation for Rockchip PCIe PHY

Rockchip PCIE PHY

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Required properties:

 - compatible: rockchip,rk3399-pcie-phy

 - #phy-cells: must be 0

 - clocks: Must contain an entry in clock-names.

	See ../clocks/clock-bindings.txt for details.

 - clock-names: Must be "refclk"

 - resets: Must contain an entry in reset-names.

	See ../reset/reset.txt for details.

 - reset-names: Must be "phy"

Example:

grf: syscon@ff770000 {

	compatible = "rockchip,rk3399-grf", "syscon", "simple-mfd";

	#address-cells = <1>;

	#size-cells = <1>;

	...

	pcie_phy: pcie-phy {

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

		#phy-cells = <0>;

		clocks = <&cru SCLK_PCIEPHY_REF>;

		clock-names = "refclk";

		resets = <&cru SRST_PCIEPHY>;

		reset-names = "phy";

	};

};