dp8393x: replace PROP_PTR with PROP_LINK

dp8393x: replace PROP_PTR with PROP_LINK

Link property is the correct way to pass a MemoryRegion to a device
for DMA purposes.

Sidenote: as a sysbus device, this remains non-usercreatable
even though we can drop the specific flag here.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>