tools/power turbostat: rename num_cores_per_pkg to num_cores_per_node

turbostat incorrectly assumes that there is one node per package.  As a
result num_cores_per_pkg is not correctly named and is actually
num_cores_per_node.

Rename num_cores_per_pkg to num_cores_per_node.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>