hwpstate_amd(4): Rename CPPC register macros

To be closer to AMD's official terminology, except for the "Lowest
Non-Linear Performance" field which we label as 'EFFICIENT_PERF' closer
to Intel's ("Most Efficient Performance"), and to clear possible
confusion.

No functional change (intended).

Reviewed by: aokblast
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54998