PM: EM: Introduce em_dev_update_perf_domain() for EM updates

Add API function em_dev_update_perf_domain() which allows the EM to be
changed safely.

Concurrent updaters are serialized with a mutex and the removal of memory
that will not be used any more is carried out with the help of RCU.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>