Add support for Buffalo WZR-HP-AG300H atheros MIPS router.

Special thanks to Nicholas Esborn for the loaner router to get this
target bootstrapped.

Review:  D777
Reviewed by:    adrian
Sponsored by:   Nicholas Esborn <nick@desert.net>


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275057 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f