hw/cpu: introduce CPU clusters

hw/cpu: introduce CPU clusters

This commit adds the cpu-cluster type. It aims at gathering CPUs from
the same cluster in a machine.

For now it only has a `cluster-id` property.

Documentation in cluster.h written with the help of Peter Maydell.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20181207090135.7651-2-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>