mtd: nand: Introduce CONFIG_SYS_NAND_BUSWIDTH_16BIT

Introduce CONFIG_SYS_NAND_BUSWIDTH_16BIT option so that other NAND controller
drivers could use it when a 16-bit NAND is deployed.

drivers/mtd/nand/ndfc has CONFIG_SYS_NDFC_16BIT, so just rename it, so that
other NAND drivers could reuse the same symbol.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>