mtd: rawnand: Remove forward declaration of mtd_info

struct mtd_info is defined in linux/mtd/mtd.h which is included
at the beginning of nand_base.c, there's thus no need for the
forward declaration of mtd_info.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>