boards: nxp: fix s26ks512s0 flash write-block-size

- Sets s26ks512s0 flash write-block-size to correct 256KB.
- Optimizes MCUboot partitions to fit the correct write-block-size.

Fixes #80284

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>
(cherry picked from commit a730d9ab)