sched/rt: take cumulative window demand into consideration

When all other conditions are same, by placing task on the least
cumulative window demand CPU, we can restrain CPU frequency and
reduce energy cost.

Select the least cumulative window demand CPU when in such case.

Change-Id: Id9ea531d67cfd6d560c8b99dd52e882f912b96a9
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>