usb: gadget: add tracepoints to the gadget API

This new set of tracepoints will help all gadget
drivers and UDC drivers when problem appears. Note
that, in order to be able to add tracepoints to
udc-core.c we had to rename that to core.c and
statically link it with trace.c to form
udc-core.o. This is to make sure that module name
stays the same.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>