peripheral drivers are using usb_add_gadget()/usb_del_gadget() to
register/unregister to the udc-core.

The udc-core will take the first available gadget driver and attach
function driver which is calling usb_gadget_register_driver(). This is
the same behaviour we have right now.

Only dummy_hcd was tested, the others were compiled tested.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Anton Tikhomirov <av.tikhomirov@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Dan Carpenter <error27@gmail.com>
Cc: Darius Augulis <augulis.darius@gmail.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Li Yang <leoli@freescale.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: NMike Frysinger <vapier@gentoo.org>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Pavankumar Kondeti <pkondeti@codeaurora.org>
Cc: Roy Huang <roy.huang@analog.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Toshiharu Okada <toshiharu-linux@dsn.okisemi.com>
Cc: Xiaochen Shen <xiaochen.shen@intel.com>
Cc: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Cc: Yuan-Hsin Chen <yhchen@faraday-tech.com>
Cc: cxie4 <cxie4@marvell.com>
Cc: linux-geode@lists.infradead.org
Signed-off-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>