x86: add AMD RAPL MSR definitions to specialreg.h

[afscoelho_gmail.com]

Summary

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.