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 can
estimate power using the P = C * V^2 * f equation where C, V, and f
respectively are the capacitance of the CPU and the voltage and frequency
of the OPP.

The CPU capacitance is read from the "dynamic-power-coefficient" DT
binding, and the voltage and frequency values are obtained from PM_OPP.

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