sched: hmp: unify active migration code

sched: hmp: unify active migration code

The HMP active migration code is functionally identical to the CFS
active migration code apart from one flag check. Share the code
and make the flag check optional.

Two wrapper functions allow the flag check to be present or not.

Thanks to tixy@linaro.org for pointing out the build break and a
good solution in an earlier version.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>
[ Rebased on top of 1.1-Juno-rc0 ]
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>