cris: nand: use the mtd instance embedded in struct nand_chip

struct nand_chip now embeds an mtd device. Patch all drivers to make use
of this mtd instance instead of using the instance embedded in their
private struct or dynamically allocated.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>