extcon: usb-gpio: Add VBUS detection support

Driver can now work with both ID and VBUS pins or either one of
them.

There can be the following 3 cases

1) Both ID and VBUS GPIOs are available:

ID = LOW -> USB_HOST active, USB inactive
ID = HIGH -> USB_HOST inactive, USB state is same as VBUS.

2) Only ID GPIO is available:

ID = LOW -> USB_HOST active, USB inactive
ID = HIGH -> USB_HOST inactive, USB active

3) Only VBUS GPIO is available:

VBUS = LOW -> USB_HOST inactive, USB inactive
VBUS = HIGH -> USB_HOST inactive, USB active

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>