Set the caching mode for the usermode mapping of the HPET registers
page to uncached.

Reviewed by:	rpaulo
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273647 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f