mtd: nand: jz4740: switch to mtd_ooblayout_ops

Implementing the mtd_ooblayout_ops interface is the new way of exposing
ECC/OOB layout to MTD users.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>