doc/devicetree: Persistent memory region bindings

Add device-tree binding documentation for the nvdimm region driver.

Cc: devicetree@vger.kernel.org
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>