mtd: spi-nor: EXPORT symbols which could be used by module drivers

Fix errors like this:

    ERROR: "spi_nor_ids" [drivers/mtd/devices/m25p80.ko] undefined!
    ERROR: "spi_nor_scan" [drivers/mtd/devices/m25p80.ko] undefined!
    make[1]: *** [__modpost] Error 1
    make: *** [modules] Error 2

Signed-off-by: Brian Norris <computersforpeace@gmail.com>