PM: EM: Clarify abstract scale usage for power values in Energy Model

PM: EM: Clarify abstract scale usage for power values in Energy Model

The Energy Model (EM) can store power values in milli-Watts or in abstract
scale. This might cause issues in the subsystems which use the EM for
estimating the device power, such as:

 - mixing of different scales in a subsystem which uses multiple
   (cooling) devices (e.g. thermal Intelligent Power Allocation (IPA))

 - assuming that energy [milli-Joules] can be derived from the EM power
   values which might not be possible since the power scale doesn't have
   to be in milli-Watts

To avoid misconfiguration add the requisite documentation to the EM and
related subsystems: EAS and IPA.

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