sched/cpufreq: Hook em_pd_get_higher_power() into get_next_freq()

Choose the highest OPP for a given energy cost, allowing to skip lower
frequencies that would not be cheaper in terms of consumed power. These
frequencies can still be interesting to keep in the energy model to give
more freedom to thermal throttling, but should not be selected under
normal circumstances.

This also prepares the ground for energy-aware frequency boosting.

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