powercap / RAPL: handle atom and core differences

RAPL implementation on Atom has made some changes that are not compatible
with Core CPUs. Specifically, it is different in the way units are computed
as well as floor frequency is enforced.

This patch uses the per CPU model functions to handle the differences. Intel
Software Developers' Manual has also been updated to reflect the changes in
Table 35-7 V3C.

Signed-off-by: Ajay Thomas <ajay.thomas.david.rajamanickam@intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>