cpufreq: scmi: 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 scmi-cpufreq driver which uses
the power costs provided by the firmware.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Change-Id: I2e6036acbf004d41f921e1396983b07e022a5399