cpufreq(4): Add support for Intel Speed Shift

Intel Speed Shift is Intel's technology to control frequency in hardware,
with hints from software.

Let's get a working version of this in the tree and we can refine it from
here.

Submitted by: bwidawsk, scottph
Reviewed by: bcr (manpages), myself
Discussed with: jhb, kib (earlier versions)
With feedback from: Greg V, gallatin, freebsdnewbie AT freenet.de
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D18028