sched: walt: fix cluster sorting

The power_cost() returns the same value for all CPUs when HMP
is not defined. Due to this clusters are sorted incorrectly.
When HMP is not defined, use max_possible_capacity as the
power cost.

Change-Id: Ifcee551f4bc3a9a9ca938fcb24d04d5a08cddb98
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>