mtd: nand: NULL terminate a of_device_id table

of_device_id tables should be NULL terminated.

Fixes: 07b23e3db9ed ("mtd: nand: Cleanup/rework the atmel_nand driver")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>