mtd: spi-nor: make sector_size optional

Most of the (old, non-SFDP) flashes use a sector size of 64k. Make that
a default value so it can be optional in the flash_info database.

As a preparation for conversion to the new database format, set the
sector size to zero if the default value is used. This way, the actual
change is happening with this patch ant not with a later conversion
patch.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/20230807-mtd-flash-info-db-rework-v3-10-e60548861b10@kernel.org


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