Allwinner USB DRD support (musb_otg)

Allwinner USB DRD is based on the Mentor USB OTG controller, with a
different register layout and a few missing registers.

The code is by Andrew Turner (andrew).

Reviewed by: hselasky, manu
Obtained from: andrew
MFC after: 5 weeks
Differential Revision: https://reviews.freebsd.org/D5881