libnvdimm, of_pmem: use dev_to_node() instead of of_node_to_nid()

Remove the direct dependency on of_node_to_nid() by using dev_to_node()
instead. Any DT platform device will have its NUMA node id set when the
device is created.

With this, commit 291717b6 ("libnvdimm, of_pmem: workaround OF_NUMA=n
build error") can be reverted.

Fixes: 71719760 ("libnvdimm: Add device-tree based driver")
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Oliver O'Halloran <oohall@gmail.com>
Cc: linux-nvdimm@lists.01.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>