sched/walt: Improve the scheduler

This change is for general scheduler improvement.

Change-Id: If03bf5f104740adf09d854837ae9b141dc97cbc6
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>
[pkondeti@codeaurora: Added tunable based on priority]
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>