usb: Convert vbus-gpio usage to new devicetree macros

Convert the usb_vbus_set function to utilize a chosen property
('zephyr,usb-device') to determine the device node that should have a
'vbus-gpios' property set for usb_vbus_set to be more than just a noop.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>