sched: WALT: introduce sched_group_{up,down}_migrate knobs

Introduce sched_group_upmigrate and sched_group_downmigrate to control
these tunables at a runtime.

Change-Id: I8d88b0ca15dc1daec99b03de116192acc2660ba7
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>