Documentation: bindings: add dt documentation for Rockchip PCIe PHY

This patch adds a binding that describes the Rockchip PCIe PHY found
on Rockchip SoCs PCIe interface.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

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

+Rockchip PCIE PHY
+-----------------------
+
+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";
+	};
+};