usb: gadget: Support endpoint allocation by ep name

Implement support for allocating an EP based on name.
This allows a function driver to request an EP using an
ep name string. This is useful to request GSI specific
EPs.

CRs-Fixed: 946385

Change-Id: Id38e6e1e3c2d82f440c0507d24f18f808dc3e4dc
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>