sched: Take cluster's minimum power into account for optimizing sbc()

The select_best_cpu() algorithm iterates over all the clusters and
selects the most power efficient CPU that satisfies the task needs.
During the search, skip the next cluster if its minimum power cost
is higher than the power cost of an eligible CPU found in the previous
cluster.

In a b.L system, if the BIG cluster minimum power cost is higher than
the maximum power cost of the little cluster, this optimization avoids
searching the BIG cluster if an eligible CPU is found in the little
cluster.

Change-Id: I5e3755f107edb6c72180edbec2a658be931c276d
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>