Documentation: Document Hisilicon hix5hd2 sata PHY

Add necessary binding documentation SATA PHY on Hisilicon hix5hd2 soc.

Signed-off-by: Jiancheng Xue <[e-mail]>
Signed-off-by: Zhangfei Gao <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

Documentation/devicetree/bindings/phy/hix5hd2-phy.txt

···
+Hisilicon hix5hd2 SATA PHY
+-----------------------
+
+Required properties:
+- compatible: should be "hisilicon,hix5hd2-sata-phy"
+- reg: offset and length of the PHY registers
+- #phy-cells: must be 0
+Refer to phy/phy-bindings.txt for the generic PHY binding properties
+
+Optional Properties:
+- hisilicon,peripheral-syscon: phandle of syscon used to control peripheral.
+- hisilicon,power-reg: offset and bit number within peripheral-syscon,
+	register of controlling sata power supply.
+
+Example:
+	sata_phy: phy@f9900000 {
+		compatible = "hisilicon,hix5hd2-sata-phy";
+		reg = <0xf9900000 0x10000>;
+		#phy-cells = <0>;
+		hisilicon,peripheral-syscon = <&peripheral_ctrl>;
+		hisilicon,power-reg = <0x8 10>;
+	};