sched: walt: Sort the clusters based on the energy-model

Make use of the cluster level energy information for sorting
the clusters. Currently we assume power is inversely proportional
to the capacity which may not necessarily be true for all systems.

Change-Id: I2f3881121148076d90bd7c83d8deb9225e870cf6
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>