Documentation: dt: usb: Add compatible strings for USB DBM

The current bindings documentation for the USB DBM device do
not explicitly list the actual supported compatible strings.
Add them in order to satisfy checkpatch.

Change-Id: Icb4d2d76ab42b2474584a8345e0098dce35d000d
Signed-off-by: Jack Pham <jackp@codeaurora.org>