mtd: spi-nor: Move Software Write Protection logic out of the core

It makes the core file a bit smaller and provides better separation
between the Software Write Protection features and the core logic.
All the next generic software write protection features (e.g. Individual
Block Protection) will reside in swp.c.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Acked-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210322075131.45093-2-tudor.ambarus@microchip.com