Allwinner a83t: enable USB support

Originally a patch by Mark Millard, augmented with information from work
done on NetBSD by jmcneill@.

Submitted by: Mark Millard (markmi@dsl-only.net)
Reviewed by: emaste, manu
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D13240