AMD SMU driver (amdsmu)

Hi!

I have added an amdsmu driver for the AMD SMU chip (which is necessary
for S0i3 entry as well as diagnostics), and I'm looking for people to
review and test these changes out.

The three patches I'm interested in at the moment are:

- https://reviews.freebsd.org/D48683 amdsmu: Initial work on a driver
for the AMD SMU
- https://reviews.freebsd.org/D48714 amdsmu: Get and expose sysctls
for metrics about last sleep
- https://reviews.freebsd.org/D51487 amdsmu: Read and dump idlemask

There is one other amdsmu-related patch, but it depends on ACPI
changes for s2idle that haven't been committed yet, so it's less of a
priority:

- https://reviews.freebsd.org/D48721 amdsmu: Sleep entry/exit hints for PMFW

This is currently only expected to work on the following CPUs:

    #define CPUID_AMD_REMBRANDT    0x14B5
    #define CPUID_AMD_PHOENIX    0x14E8
    #define CPUID_AMD_STRIX_POINT    0x14A4

And of these, I have only had the opportunity to test on Phoenix
(specifically the 7040 series).

Thanks!
Aymeric