sched: rt: Fix start cpu for RT energy aware task placement

For RT tasks, energy aware task placement starts with highest level
sched_domain of prev_cpu. If the task's prev_cpu is higher capacity,
then it will start with the group that contains the higher capacity cpus
and so will visit the lower capacity cpus later. Not only that - it may
skip the lower capacity cpus if the higher capacity cpus are found
to be lower utilization.

This is not power friendly - a rt task that could easily fit on lower
capacity cpus will be placed on higher capacity cpus just because its
prev cpu happened to be one.

Fix this, by using min capacity cpu as start cpu for RT energy aware
task placement decisions.

Change-Id: Icd8550eccdc7e6f847a5b0c06b6944c85d26bd52
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>