Documentation: EM: Add artificial EM registration description

Add description about new artificial EM registration and use cases.
Update also the documentation with the new .get_cost() callback
description and usage.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>