powercap / RAPL: abstract per cpu type functions

RAPL implementations may vary slightly between Core and Atom CPUs. There are
also potential differences among generations of CPUs within the family.

This patch adds a per model structure to abstract the differences such that
variations can be handled cleanly.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>