sched: Scheduler pl enhancement

This change is for general scheduler improvement.

Change-Id: I5ec4bc83c1af2a3edaa047296c505144bd554073
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
[joonwoop: Added SCHED_CPUFREQ_WALT flag and resolved conflict.]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>