sched/idle: Enter wfi state instead of polling during active migration

WFI's wakeup latency is low enough, use that instead of polling and
burning power.

Change-Id: Iee1c1cdf515224267925037a859c6a74fc61abb7
Signed-off-by: Kazuki H <kazukih0205@gmail.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>