mtd: rawnand: allocate model parameter dynamically

Thanks to the migration of all drivers to use nand_scan() and the
related nand_controller_ops, we can now allocate data during the
detection phase. Let's do it first for the NAND model parameter which
is allocated in nand_detect().

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