tsc: AMD Family 17h, 19h: compute frequency from Core::X86::Msr:PStateDef

tsc: AMD Family 17h, 19h: compute frequency from Core::X86::Msr:PStateDef

Compute the TSC frequency on AMD family 17h and 19h CPUs using the
PStateDef MSRs.

   Link 1: https://marc.info/?l=openbsd-tech&m=166394236029484&w=2
   Link 2: https://marc.info/?l=openbsd-tech&m=166446065916283&w=2
Test list: https://marc.info/?l=openbsd-tech&m=166646389821326&w=2

Reviewed by kettenis@ using the AMD documents cited in the comments.
Maybe reviewed by mlarkin@?  I can't remember.  He seemed supportive
of the idea at least.

ok kettenis@