cpufreq: scpi: Register an Energy Model

The Energy Model framework provides an API to register the active power
of CPUs. This commit calls this API from the scpi-cpufreq driver which
uses the power estimation helper from PM_OPP.

Change-Id: I113fa2edf8201c1272c9fb5a0c6c39622ae53f94
Signed-off-by: Quentin Perret <quentin.perret@arm.com>