mtd: spi-nor: Drop the MFR definitions

Cross manufacturer code is unlikely and discouraged, get rid of the
MFR definitions.

Suggested-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>