dt-bindings: phy: add support for STM32 USB PHY Controller (USBPHYC)

This patch adds the device tree bindings description for STM32 USBPHYC
(USB PHY Controller).

Signed-off-by: Amelie Delaunay <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

Documentation/devicetree/bindings/phy/phy-stm32-usbphyc.txt

···
+STMicroelectronics STM32 USB HS PHY controller
+
+The STM32 USBPHYC block contains a dual port High Speed UTMI+ PHY and a UTMI
+switch. It controls PHY configuration and status, and the UTMI+ switch that
+selects either OTG or HOST controller for the second PHY port. It also sets
+PLL configuration.
+
+USBPHYC
+      |_ PLL
+      |
+      |_ PHY port#1 _________________ HOST controller
+      |                    _                 |
+      |                  / 1|________________|
+      |_ PHY port#2 ----|   |________________
+      |                  \_0|                |
+      |_ UTMI switch_______|          OTG controller
+
+
+Phy provider node
+=================
+
+Required properties:
+- compatible: must be "st,stm32mp1-usbphyc"
+- reg: address and length of the usb phy control register set
+- clocks: phandle + clock specifier for the PLL phy clock
+- #address-cells: number of address cells for phys sub-nodes, must be <1>
+- #size-cells: number of size cells for phys sub-nodes, must be <0>
+
+Optional properties:
+- assigned-clocks: phandle + clock specifier for the PLL phy clock
+- assigned-clock-parents: the PLL phy clock parent
+- resets: phandle + reset specifier
+
+Required nodes: one sub-node per port the controller provides.
+
+Phy sub-nodes
+==============
+
+Required properties:
+- reg: phy port index
+- phy-supply: phandle to the regulator providing 3V3 power to the PHY,
+	      see phy-bindings.txt in the same directory.
+- vdda1v1-supply: phandle to the regulator providing 1V1 power to the PHY
+- vdda1v8-supply: phandle to the regulator providing 1V8 power to the PHY
+- #phy-cells: see phy-bindings.txt in the same directory, must be <0> for PHY
+  port#1 and must be <1> for PHY port#2, to select USB controller
+
+
+Example:
+		usbphyc: usb-phy@5a006000 {
+			compatible = "st,stm32mp1-usbphyc";
+			reg = <0x5a006000 0x1000>;
+			clocks = <&rcc_clk USBPHY_K>;
+			resets = <&rcc_rst USBPHY_R>;
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			usbphyc_port0: usb-phy@0 {
+				reg = <0>;
+				phy-supply = <&vdd_usb>;
+				vdda1v1-supply = <&reg11>;
+				vdda1v8-supply = <&reg18>
+				#phy-cells = <0>;
+			};
+
+			usbphyc_port1: usb-phy@1 {
+				reg = <1>;
+				phy-supply = <&vdd_usb>;
+				vdda1v1-supply = <&reg11>;
+				vdda1v8-supply = <&reg18>
+				#phy-cells = <1>;
+			};
+		};