sched: EAS: decouple capacity margin

Current consolidated capacity_margin is used by frequency guidance
as well as task placement.  This makes impossible to utilize little
CPU more than capacity margin as all the tasks which have more than
the margin will be upmigrated even if the task fits in little CPU.

Decouple capacity_margin into capacity_margin for CPU frequency
guidance and task placement.  We want to have higher margin for
former and lower margin for latter to raise frequency and account
task demand.

Also, introduce capacity_margin_down which controls down migration
threshold.  With single capacity_margin tipping point, any task
which is at the capacity margin can oscillate between little and big
cluster.

Change-Id: I3651b2bb800335807fc6472676bf29cd357df303
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>