PM / OPP: cpufreq-dt: Move power estimation function

The cpufreq-dt driver has a function estimating the power of CPUs using
the dynamic-power-coefficient DT binding and the parameters of PM_OPP.
Since this function can be useful to other drivers, relocate it to
PM_OPP which is already a dependency anyway.

Change-Id: I34f8f9cd9433c622c82f23f32ae9968a096a4390
Signed-off-by: Quentin Perret <quentin.perret@arm.com>