mtd: spi-nor: get rid of non-power-of-2 page size handling

The Xilinx flashes were the only users of page sizes that were not power
of 2. Support for them has been dropped, thus we can also get rid of the
special page size handling for it.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Acked-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
[pratyush@kernel.org: fixup minor typos and grammar in commit message]
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20240419141249.609534-3-mwalle@kernel.org