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

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