PM: Introduce em_pd_get_higher_freq()

em_pd_get_higher_freq() returns a frequency greater or equal to the
provided one while taking into account a given cost margin. It also
skips inefficient OPPs that have a higher cost than another one with a
higher frequency.

The efficiency of an OPP is measured as efficiency=capacity/power.
OPPs with the same efficiency are assumed to be equivalent, since they
will consume as much energy for a given amount of work to do. That may
take more or less time depending on the frequency, but will consume the
same energy.

Signed-off-by: Douglas RAILLARD <douglas.raillard@arm.com>