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>
Change-Id: I5082959fb71679ffeafc3a8db48a714b8617467b
Git-commit: 541332a13b1ded42097ba96c52c7bc70931e528c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[jackp@codeaurora.org: Fixed checkpatch.pl style warnings]
Signed-off-by: Jack Pham <jackp@codeaurora.org>