hw/dma: Do not build the xlnx_dpdma device for the MicroBlaze machines

hw/dma: Do not build the xlnx_dpdma device for the MicroBlaze machines

The xlnx_dpdma device is only used by the ZynqMP AArch64 machine
(not the MicroBlaze PMU). Remove it from the ZynqMP generic objects.
(Note, this entry was duplicated for the AArch64).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190427141459.19728-4-philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>