arch_topology: Set cluster identifier in each core/thread from /cpu-map

Let us set the cluster identifier as parsed from the device tree
cluster nodes within /cpu-map.

We don't support nesting of clusters yet as there are no real hardware
to support clusters of clusters.

Tested-by: Ionela Voinescu <ionela.voinescu@arm.com>
Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>