Add helper function to generate dsdt aml code for the gpex pci host.
Largely copied from arm/virt.  Configuration is handled by passing
a config struct instead of looked up from memory map.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 20200928104256.9241-3-kraxel@redhat.com