Documentation: bindings: add doc for the USB2 ChipIdea USB driver

Document the USB2 ChipIdea driver (ci13xxx) bindings.
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

+* USB2 ChipIdea USB controller for ci13xxx
+
+Required properties:
+- compatible: should be "chipidea,usb2"
+- reg: base address and length of the registers
+- interrupts: interrupt for the USB controller
+
+Optional properties:
+- clocks: reference to the USB clock
+- phys: reference to the USB PHY
+- phy-names: should be "usb-phy"
+- vbus-supply: reference to the VBUS regulator
+
+Example:
+
+	usb@f7ed0000 {
+		compatible = "chipidea,usb2";
+		reg = <0xf7ed0000 0x10000>;
+		interrupts = <GIC_SPI 11 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&chip CLKID_USB0>;
+		phys = <&usb_phy0>;
+		phy-names = "usb-phy";
+		vbus-supply = <&reg_usb0_vbus>;
+	};