sched: Add migration support for multi cluster systems

In case of multi cluster systems with varying capacity of CPUs,
need for migration of tasks would be different as task would need
to get migrated to CPU in adjacent cluster. This change adds
support to accept multiple values for the {up,down}_migrate knobs,
so that, tasks can be migrated to CPU in adjacent cluster.

Change-Id: I325cc71884d9bbac14475cd838a3955d53e03d1e
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>