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 "extcon-gpio"
- extcon-id: The unique id of specific external connector.
	     Valid range is 0 (EXTCON_NONE) to 63 (EXTCON_NUM).
	     Refer include/linux/extcon.h for details.
- gpio: Specify GPIO (see gpio binding)
- debounce-ms: Debounce time for GPIO IRQ in ms
- irq-flags: interrupt flags (edge/level). Refer to "include/dt-bindings/interrupt-controller/irq.h"
- pinctrl-names, pinctrl-0, pinctrl-1,.. pinctrl-n: Refer to "Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt"
  for these optional properties

Example:
	extcon_storage_cd {
		compatible = "extcon-gpio";
		extcon-id = <62>; /* EXTCON_MECHANICAL */
		gpio = <&tlmm 126 GPIO_ACTIVE_LOW>;
		debounce-ms = <200>;
		irq-flags = <IRQ_TYPE_EDGE_BOTH>;
	}

	&ufshc_card {
		extcon = <&extcon_storage_cd>;
	};

	&sd_card {
		extcon = <&extcon_storage_cd>;
	};