usb: dwc3-msm: dbm: Merge dbm-1_5.c into dbm.c

Combine into a single file. While at it we can remove the
usb_add_dbm() function as well as simplify the list management.
Remove the unused index parameter to usb_get_dbm_by_phandle()
since it is always 0.

Change-Id: I2c94127a6907b386e4b31796f01b119cd8062af9
Signed-off-by: Jack Pham <jackp@codeaurora.org>