USB: gadget: qti: Add qti_usb gadget

Configfs based gadget relies on userspace to specify configfs
attributes in order to enable USB composition. This causes USB
enumeration to not happen until userspace comes up.
However, if there is no dependency on userspace then there
is no reason to use configfs and by having an in-kernel USB
gadget we can get USB enumerated much early.

Change-Id: I5feec52ff45c82a166fab73d0d1c586254c7025f
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>