powercap/intel rapl: Convert to hotplug state machine

Install the callbacks via the state machine as a first step. The init/exit
code is a duplicate of the hotplug code. This is cleaned up in a
consecutive patch.

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