usb: udc: add usb_udc_vbus_handler

This commit updates udc core vbus status, and try to connect
or disconnect gadget.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>