mtd: nand: gpmi: 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: Han Xu <han.xu@nxp.com>
Tested-by: Han Xu <han.xu@nxp.com>