sched: precompute required frequency for CPU load

At present in order to estimate power cost of CPU load, HMP scheduler
converts CPU load to coresponding frequency on the fly which can be
avoided.

Optimize and reduce execution time of select_best_cpu() by precomputing
CPU load to frequency conversion.  This optimization reduces about ~20% of
execution time of select_best_cpu() on average.

Change-Id: I385c57f2ea9a50883b76ba6ca3deb673b827217f
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>