amdtemp(4): Add support for Family 17h temperature sensor

The sensor value is formatted similarly to previous models (same
bitfield sizes, same units), but must be read off of the internal
System Management Network (SMN) from the System Management Unit (SMU)
co-processor.

PR: 218264
Reported and tested by: Nils Beyer <nbe AT renzel.net>
Reviewed by: avg (no +1), mjoras, truckman
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12217