USB GPIO Extcon device

This is a virtual device used to generate USB cable states from the USB ID pin
connected to a GPIO pin.

Required properties:
- compatible: Should be "linux,extcon-usb-gpio"

Either one of id-gpio or vbus-gpio must be present. Both can be present as well.
- id-gpio: gpio for USB ID pin. See gpio binding.
- vbus-gpio: gpio for USB VBUS pin.

Example: Examples of extcon-usb-gpio node in dra7-evm.dts as listed below:
	extcon_usb1 {
		compatible = "linux,extcon-usb-gpio";
		id-gpio = <&gpio6 1 GPIO_ACTIVE_HIGH>;
	}

	&omap_dwc3_1 {
		extcon = <&extcon_usb1>;
	};
