ARM64: Enable multi-core scheduler support by default

Make sure that the task scheduler domain hierarchy is set-up correctly
on systems with single or multi-cluster topology.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>