powercap: DTPM: Check for Energy Model type

The Energy Model power values might be artificial. In such case
it's safe to bail out during the registration, since the PowerCap
framework supports only micro-Watts.

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>