mtd: rawnand: use mtd_device_register() where applicable

If driver doesn't specify parsers it can use that little helper.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Xiaolei Li <xiaolei.li@mediatek.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>