usb: renesas_usbhs: Add device tree bindings documentation

Document the device tree bindings for the Renesas USBHS controller.
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/renesas_usbhs.txt

+Renesas Electronics USBHS driver
+
+Required properties:
+  - compatible: Must contain one of the following:
+	- "renesas,usbhs-r8a7790"
+	- "renesas,usbhs-r8a7791"
+  - reg: Base address and length of the register for the USBHS
+  - interrupts: Interrupt specifier for the USBHS
+  - clocks: A list of phandle + clock specifier pairs
+
+Optional properties:
+  - renesas,buswait: Integer to use BUSWAIT register
+  - renesas,enable-gpio: A gpio specifier to check GPIO determining if USB
+			 function should be enabled
+  - phys: phandle + phy specifier pair
+  - phy-names: must be "usb"
+
+Example:
+	usbhs: usb@e6590000 {
+		compatible = "renesas,usbhs-r8a7790";
+		reg = <0 0xe6590000 0 0x100>;
+		interrupts = <0 107 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&mstp7_clks R8A7790_CLK_HSUSB>;
+	};