acpi nvdimm: fix device physical address base

According to ACPI 6.0  spec, "Memory Device Physical Address
Region Base" in memdev is defined as "This field provides the
Device Physical Address base of the region". This field should
be zero in our case

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
