usb: gadget: add reset API at usb_gadget_driver

Adding reset API for UDC bus reset handler is useful for below
two issues.

Current disconnect API at usb_gadget_driver is also invoked at
udc's bus reset handler, but the document says it is invoked when
the host is disconnected.

Besides, we may expect the gadget_driver to do different things
for host sends bus reset and host disconnects gadget, eg, we may not
want to flush dirty page for mass storage at bus reset, and want to
do it at disconnection.

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