powercap/rapl: add package reference per domain

This patch adds to each rapl domain a reference of the package
it belongs to. At runtime, we can then avoid searching the package
data for each access. It simplifies the domain level operations
which depend on package level information.

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