usb: gadget: add 'ep_match' callback to usb_gadget_ops

Add callback that is called by epautoconf to allow UDC driver match the
best endpoint for specific descriptor. It's intended to supply mechanism
which allows to get rid of chip-specific endpoint matching code from
epautoconf.

If gadget has set 'ep_match' callback we prefer to call it first, and
if it fails to find matching endpoint, then we try to use default matching
algorithm.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>