libnvdimm: Add device-tree based driver

This patch adds peliminary device-tree bindings for persistent memory
regions. The driver registers a libnvdimm bus for each pmem-region
node and each address range under the node is converted to a region
within that bus.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>