mtd: nand: omap_gpmc: rename struct nand_bch_priv to struct omap_nand_info

This patch renames 'struct nand_bch_priv' which currently holds private data only
for BCH ECC schemes, into 'struct omap_nand_info' so that same can be used for
all ECC schemes

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Stefan Roese <sr@denx.de>