mtd: rawnand: Declare the nand_manufacturer structure out of nand_chip

Now that struct nand_manufacturer type is free, use it to store the
nand_manufacturer_desc and the manufacturer's private data.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-6-miquel.raynal@bootlin.com