sound: Include ai2s and davbus for PowerPC

Include PowerPC ai2s and davbus sound modules in the metadriver.

Reviewed by: christos, emaste
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47467

(cherry picked from commit cdd3234186b19e1b9d7cfc985ef8d247552b0f11)