extcon: usb-gpio: Introduce gpio usb extcon driver

This driver observes the USB ID pin connected over a GPIO and
updates the USB cable extcon states accordingly.

The existing GPIO extcon driver is not suitable for this purpose
as it needs to be taught to understand USB cable states and it
can't handle more than one cable per instance.

For the USB case we need to handle 2 cable states.
1) USB (attach/detach)
2) USB-HOST (attach/detach)

This driver can be easily updated in the future to handle VBUS
events in case it happens to be available on GPIO for any platform.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>