PM: EM: Update Energy Model with new flag indicating power scale

Update description and meaning of a new flag, which indicates the type of
power scale used for a registered Energy Model (EM) device.

Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>