usb: chipidea: otg: gadget module load and unload support

This patch is to support load and unload gadget driver in full OTG mode.

Signed-off-by: Li Jun <jun.li@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Tested-by: Jiada Wang <jiada_wang@mentor.com>
Cc: <stable@vger.kernel.org> #v4.0+