sched/topology: Mark SD_OVERLAP as SDF_NEEDS_GROUPS

A sched_domain can only have overlapping sched_groups if it has more than
one group.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20200817113003.20802-16-valentin.schneider@arm.com