USB: gadget: pxa2xx_udc supports inverted vbus

Some boards (like e.g. Tosa) invert the VBUS-detection signal:
it's low when a host is supplying VBUS, and high otherwise.
Allow specifying whether gpio_vbus value is inverted.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>