mtd: rawnand: cs553x: Declare controllers instead of NAND chips

The CS553x companion chip embeds 4 NAND controllers. Declare them as
NAND controllers instead of NAND chips. That's done in preparation
of the transition to exec_op().

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200501090650.1138200-2-boris.brezillon@collabora.com