mtd: spi-nor: core: Allow flashes to specify MTD writesize

mtd: spi-nor: core: Allow flashes to specify MTD writesize

Some flashes like the Cypress S28 family use ECC. Under this ECC scheme,
multi-pass writes to an ECC block is not allowed. In other words, once
data is programmed to an ECC block, it can't be programmed again without
erasing it first.

Upper layers like file systems need to be given this information so they
do not cause error conditions on the flash by attempting multi-pass
programming. This can be done by setting 'writesize' in 'struct
mtd_info'.

Set the default to 1 but allow flashes to modify it in fixup hooks. If
more flashes show up with this constraint in the future it might be
worth it to add it to 'struct flash_info', but for now increasing its
size is not worth it.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201201102711.8727-3-p.yadav@ti.com