libnvdimm, pmem: move pmem to drivers/nvdimm/

Prepare the pmem driver to consume PMEM namespaces emitted by regions of
an nvdimm_bus instance.  No functional change.

Acked-by: Christoph Hellwig <hch@lst.de>
Tested-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>