soc: lpc: mcxn: fix the flash_fill() fail.

- Fix flash_fill() for lpc55 and mcxnx4.
- Set FLASH_FILL_BUFFER_SIZE to the minimal size of data
  which can be written to a device (by default is only 32).
- Fix the [flash_map.test_flash_area_erase_and_flatten]
  failed test of tests/subsys/storage/flash_map.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>