sched: select backup CPU with least cumulative window demand

Choose backup CPU with the least cumulative window demand among the same
CPU's demand and C-state.  Doing such keeps CPU's frequency lower.

Change-Id: I9c1e3290146ed40d98b6d73eef60b6760eb97f1c
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>