hwpstate_amd(4): Revamp the softc

As the new CPPC and old P-states modes are exclusive, put their
respective data in a union.

Rename the field containing the content of the CPPC_REQUEST register.
It it now to be accessed using 'cppc.request'.

Use an 'unsigned int' instead of 'uint32_t' for 'flags'. This is an
internal field whose width could be changed at will. We only have one
flag at the moment.

No functional change intended.

Reviewed by: aokblast
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55004