Introduce PMCR-based cpufreq(4) driver, for IBM POWER8 and POWER9 systems

Summary: POWER8 and POWER9 use a single CPU register, per core, to change clock
speed.  Everything else is handled by the on-chip controller.  This change
necessitates a change to the cpufreq global kernel driver to bump supported
levels, as the device tree for these systems can have theoretically 256
different options.  On my POWER9 Talos, the list consists of 100 items.  At
16.67MHz intervals, that allows for a change of roughly 1.67GHz between lowest
and highest.

This has only been tested on the POWER9.  However, since they're similar, this
should work on POWER8 as well.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D15932

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