usb: gadget: introduce UDC Class

this class will be used to abstract away several of the duplicated
operations scattered among the USB gadget controller drivers.

Later, we can add an atomic notifier to tell interested drivers about
what's happening with the controller. Notifications such as suspend,
resume, enumerated, etc. will be useful, at a minimum, for implementing
usb charger detection.

As part of the converting process usb_gadget_probe_driver() is no longer
part of each udc but pushed into the ->stap() callback. The same for his
couterpart.

The core is currently set explicit to 'n'. It will be changed to 'y' once
all users are converted since it provides functions which clash with
other drivers.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>