hwpstate{_amd,intel}(4): Move common knobs to a separate file

Reason for doing this right now is to resolve the conflict on
'machdep.hwpstate_pkg_ctrl' between the Intel and AMD drivers, even
though I expect to remove it for hwpstate_amd(4) at some point.

More generally, this is going to be useful for some future code
factorization. Also, the 'debug.hwpstate_verbose' knob was moved there,
as we'll likely want to use it for the Intel driver as well (which is
currently not the case).

Note for MFC: Will be partial, since hwpstate_amd(4) does not support
CPPC in stable/15 nor stable/14.

Reviewed by: emaste
Fixes: 3e6e4e4a0d42 ("hwpstate: add CPPC support for pstate driver on AMD")
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D54528