mtd: spi-nor: move all spansion specifics into spansion.c

The clear status register flags is only available on spansion flashes.
Move all the functions around that into the spanion module.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Tested-by: Pratyush Yadav <p.yadav@ti.com> # on mt35xu512aba, s28hs512t
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-29-michael@walle.cc