sched/fair: use min capacity when evaluating idle backup cpus

When we are calculating what the impact of placing a task on a specific
cpu is, we should include the information that there might be a minimum
capacity imposed upon that cpu which could change the performance and/or
energy cost decisions.

When choosing an idle backup CPU, favour CPUs that won't end up
running at a high OPP due to a min capacity cap imposed by external
actors.

Change-Id: I566623ffb3a7c5b61a23242dcce1cb4147ef8a4a
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>