usb: gadget: Match gadget and gadget_driver according to usb_core_id

Adding new usb_core_id property which is used to match an instance
of struct usb_gadget to an instance of struct usb_gadget_driver.
This will allow constructing more then one usb periheral stack.
This new property is optional and if not specified in both the gadget
and gadget driver then it will have the default value of zero.

Change-Id: Idbab46ed76267ab4827319f2f9efe67c91b50dbd
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>

Conflicts:
	drivers/usb/gadget/udc-core.c
	include/linux/usb/gadget.h