mtd: spi-nor: export more functions to be used in vendor modules

We will move vendor specific code into the vendor modules and thus we
will have to export these functions so they can be called.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-20-michael@walle.cc