mtd: rawnand: Fill memorg during detection

If we want to use the generic NAND layer, we need to have the memorg
struct appropriately filled. Patch the detection code to fill this
struct.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>