PM: EM: Add em_dev_compute_costs()

The device drivers can modify EM at runtime by providing a new EM table.
The EM is used by the EAS and the em_perf_state::cost stores
pre-calculated value to avoid overhead. This patch provides the API for
device drivers to calculate the cost values properly (and not duplicate
the same code).

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>