extcon: usb-gpio: Add support for optional VBUS output enable GPIO

On many platforms 5V VBUS output can be provided by external
circuitry and turned on via GPIO. For convenience, on micro-AB
ports this can be done in conjunction when ID pin becomes low
meaning USB_HOST mode is to be entered. Add support for an
optional "vbus-out" GPIO to this driver.

Change-Id: Id58e5dca5375b8140c75ce7cd6769dd8a31a2af7
Signed-off-by: Jack Pham <jackp@codeaurora.org>