usb: sandbox: Add gadget callbacks

Add usb_gadget_handle_interrupts(), usb_gadget_register_driver()
and usb_gadget_unregister_driver() to be able to test
binding usb gadget.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Reviewed-by: Simon Glass <sjg@chromium.org>