usb: dwc3-msm: DBM refactoring

Refactoring of DBM code to allow easy support for future
DBM versions.
All DBM specific operation are now defined in dbm.h,
and implemented in a dbm specific version module.

Note that only a single instance of DBM is currently
supported.

Change-Id: I751300240951a84a42358c3b8b5222e73c537653
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>