Add powerd(8) support for several families of AMD CPUs.

Use the same logic to calculate the nominal CPU frequency from the P-state
MSRs on family 0x12, 0x15, and 0x16 CPUs as is used for family 0x10.
Family 0x14 was included in the original patch in the PR but I left that
out as the BIOS writer's guide for family 0x14 CPUs show a different layout
for the relevant MSR and include a different formulate for calculating the
frequency.

While here, simplify a few expressions and print out the family of
unsupported CPUs in hex rather than decimal.

PR: 212020
Submitted by: Anthony Jenkins <Scoobi_doo@yahoo.com>
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D7587