amdsmu: Initial work on a driver for the AMD SMU

Start work on a driver for the AMD SMU (system management unit), which
will eventually be used for getting S0ix statistics (e.g. how long the
CPU has spent in the deepest - S0i3 - sleep state during the last
sleep) as well as letting PMFW (power management firmware, running on
the SMU) know when we intend to enter and exit sleep. It is what's
responsible for actually turning off the VDD line to the CPU.

With this commit, amdsmu is just able to get the SMU's firmware version
on AMD Rembrandt, Phoenix, and Strix Point CPUs.

This is the equivalent to amd-pmc on Linux.

Reviewed by: cem, kib, mckusick
Approved by: kib, mckusick
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48683