Do not access ISA timer if BIOS reports that there is no legacy
devices present.

On at least one machine where it would matter since the ISA timer is
power gated when booted in the UEFI mode, BIOS still reports that the
legacy devices are present. That is, user still have to manually
disable TSC calibration on such machines. Hopefully it will be more
useful in the future.

Discussed with: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed by: royger
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D16004
MFC after: 1 week