cpufreq: arm_big_little: 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.

Todo: Check if driver can handle -EPROBE_DEFER and if the call to
dev_pm_opp_get_opp_count() id realy necessary.

Change-Id: Ia808262ef6c9f2cc7819a83e8eb2f602454edfa3
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>