mtd: spi-nor: default .n_banks to 1

If .n_banks is not set in the flash_info database, the default value
should be 1. This way, we don't have to always set the .n_banks
parameter in flash_info.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230807-mtd-flash-info-db-rework-v3-8-e60548861b10@kernel.org


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>