mtd: nand: omap: add field to indicate current ecc-scheme in 'struct omap_nand_info'

Information of currently selected ECC scheme 'enum omap_ecc ecc_opt' should
available outside platform-data, so that single nand_chip->ecc callback can
support multiple ecc-scheme configurations.

Tested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>