sched/fair: Add support for need_idle task placement

Any task with PF_WAKE_UP_IDLE flag set or any task woken up
by another task with PF_WAKE_UP_IDLE flag set is placed on
an idle CPU. This only affects CPU selection with in a
selected cluster. If all the CPUs are active in the selected
cluster, the task is placed on the least loaded CPU.

Change-Id: Icf663abb60447508e9934ba92b19ffbfbbb6d3f5
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>