dt-bindings: phy: Add support for HiSilicon's hi3660 USB PHY

This patch adds binding documentation for supporting the hi3660 usb
phy on boards like the HiKey960.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Binghui Wang <wangbinghui@hisilicon.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yu Chen <chenyu56@huawei.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/phy-hi3660-usb3.txt

+Hisilicon hi3660 USB PHY
+-----------------------
+
+Required properties:
+- compatible: should be "hisilicon,hi3660-usb-phy"
+- #phy-cells: must be 0
+- hisilicon,pericrg-syscon: phandle of syscon used to control phy.
+- hisilicon,pctrl-syscon: phandle of syscon used to control phy.
+- hisilicon,eye-diagram-param: parameter set for phy
+Refer to phy/phy-bindings.txt for the generic PHY binding properties
+
+This is a subnode of usb3_otg_bc register node.
+
+Example:
+	usb3_otg_bc: usb3_otg_bc@ff200000 {
+		compatible = "syscon", "simple-mfd";
+		reg = <0x0 0xff200000 0x0 0x1000>;
+
+		usb-phy {
+			compatible = "hisilicon,hi3660-usb-phy";
+			#phy-cells = <0>;
+			hisilicon,pericrg-syscon = <&crg_ctrl>;
+			hisilicon,pctrl-syscon = <&pctrl>;
+			hisilicon,eye-diagram-param = <0x22466e4>;
+		};
+	};