mtd: nand: convert ONFI mode into data interface

struct nand_data_interface is the designated type to pass to
the NAND drivers to configure the timing. To simplify further
patches convert the onfi_sdr_timings array from type struct
nand_sdr_timings nand_data_interface.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>