powercap/rapl: track lead cpu per package

RAPL driver operates on MSRs that are under package/socket
scope instead of core scope. However, the current code does not
keep track of which CPUs are available on each package for MSR
access. Therefore it has to search for an active CPU on a given
package each time.

This patch optimizes the package level operations by tracking a
per package lead CPU during initialization and CPU hotplug. The
runtime search for active CPU is avoided.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>