Driver for CPU frequency/voltage control on the Raspberry Pi.

Differential Revision:	https://reviews.freebsd.org/D1025
Submitted by:	Daisuke Aoyama aoyama@peach.ne.jp
Reviewed by:	ian (earlier version), rpaulo
MFC after:	1 month
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275963 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f