xlnx-zynqmp: Don't create rpu-cluster if there are no RPUs

xlnx-zynqmp: Don't create rpu-cluster if there are no RPUs

If we aren't going to create any RPUs, then don't create the
rpu-cluster unit. This allows us to add an assertion to the
cluster object that it contains at least one CPU, which helps
to avoid bugs in creating clusters and putting CPUs in them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190121184314.14311-1-peter.maydell@linaro.org