mtd: spi-nor: Add post_sfdp() hook to tweak flash config

SFDP tables are sometimes wrong and we need a way to override the
config chosen by the SFDP parsing logic without discarding all of it.

Add a new hook called after the SFDP parsing has taken place to deal
with such problems.

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