By default, allow all to read the HPET registers pages. At the same
time, by, by default disallow writes to the mmaped HPET pages.

Intent is to allow userspace to use HPET as fast (i.e. no-syscall)
timecounter for gettimeofday(2). Unfortunately, the permission model
does not make it possible to safely unhide /dev/hpet in the jails even
if default mode is set to 0444, because untrusted jailed root may
change device permissions to writeable.

Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks