MFC: r267961, r309361, r322710, r323286, r326378, r326383, r326407

Sync. hwpstate with head.

r267961 (hselasky, partial):

Remove a redundant TUNABLE statement.

r309361 (danfe):

• Mention mismatching numbers in MSR vs. ACPI _PSS count warning.
• Rephrase unsupported AMD CPUs message and wrap as an overly long line.
• Improve readability when reporting resulted P-state transition (debug).

r322710, r323286 (cem):

• Add support for family 17h pstate info from MSRs.
• Yield CPU awaiting frequency change.

r326378, r326383, r326407:

• Fix some style(9) nits.
• Add a tunable "debug.hwpstate_verify" to check P-state after changing it

and turn it off by default.