mtd: spi-nor: remove .setup() callback

With the removal of the Xilinx flashes, there is no more flash driver
using that hook. The original intention was to let the driver configure
special requirements like page size an opcodes.  This is already
possible by other means and it is unlikely a flash will overwrite the
(more or less complex) setup function.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Acked-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20240419141249.609534-4-mwalle@kernel.org