sched/fair: use min capacity when evaluating placement energy costs

Add the ability to track minimim capacity forced onto a sched_group
by some external actor.

group_max_util returns the highest utilisation inside a sched_group
and is used when we are trying to calculate an energy cost estimate
for a specific scheduling scenario. Minimum capacities imposed from
elsewhere will influence this energy cost so we should reflect it
here.

Change-Id: Ibd537a6dbe6d67b11cc9e9be18f40fcb2c0f13de
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>