hwpstate_amd(4): 'epp' sysctl leaf to operate on real EPP hardware values

We were using percents, for compatibility with hwpstate_intel(4), but
this looses granularity that might be important in some scenarios or
with specific CPU models.

For consistency, hwpstate_intel(4) should be changed accordingly, at the
expense of breaking compatibility.

For release notes: Introduction of hwpstate_amd(4) deserves a release
note, even if the original commit was not tagged. Functionality
introduced by recent commits tagged with "Relnotes" should be mentioned
along that one.

PR: 292615
Reviewed by: aokblast
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55009