boards: mcxn947_qspi: fix mcuboot partition allocation

- Fixes MCUBoot partition allocation for
  frdm_mcxn947_mcxn947_cpu0_qspi
- Assign "zephyr,code-partition" in the
  frdm_mcxn947_mcxn947_cpu0_qspi overlay,
  as app.overlay is ignored if a board overlay is present.

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