sched: core: Use sched_clusters for updown migration handler

Currently, sched updown migration handler derives cluster topology
based on arch topology, the cluster information is already populated
in walt sched_cluster. So reuse it instead of deriving it again.

And move updown tunables support to under WALT.

Change-Id: Iddf4d18ddf75cc20637281d9889f671f42369513
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>