x86: add AMD RAPL MSR definitions to specialreg.h

specialreg.h is the tree's MSR registry and already carries the Intel
RAPL group. Add the AMD RAPL package/core energy and unit MSRs here so
the hwpmc RAPL class can reference them without a private driver copy.
Use the names Linux's msr-index.h gives these registers.

Reviewed by: mhorne, adrian, Ali Mashtizadeh <ali@mashtizadeh.com>
MFC after: 3 days
Sponsored by: AMD
Differential Revision: https://reviews.freebsd.org/D58027