libnvdimm: Set device node in nd_device_register

This change makes it so that we don't repeatedly overwrite the device node
for nvdimm regions. The earliest we can set the node is immediately after
calling device init, so I have moved the code there so we can avoid
rewriting the node with each uevent.

Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>