usb/gadget: Add an EP dispose() callback for EP lifetime tracking

Some UDC may want to allocate endpoints dynamically, either because
the HW supports an arbitrary large number or because (like the Aspeed
BMC SoCs), the pool of HW endpoints is shared between multiple gadgets.

The allocation side can be done rather easily using the existing
match_ep() UDC hook.

However we have no good place to "free" them.

This implements a "simple" variant of this, which calls an EP dispose
callback on all EPs associated with a gadget when the composite device
gets unbound.

This is required by my upcoming Aspeed vHub driver.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>