mtd: spi-nor: core: Introduce flash_info mfr_flags

Used in the manufacturer fixup hooks to differentiate support
between flashes of the same manufacturer. Not used in the
SPI NOR core.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20211207140254.87681-6-tudor.ambarus@microchip.com