boards: nxp: update flash code/storage allocation for MCXW72

67k of code is not enough for several BLE samples.
It makes more sense to have a large partition for code and a smaller
one for storage.
This commit reduces the storage to 16k (at the end of the flash) and
use the remaining for code.

Signed-off-by: Yassine El Aissaoui <yassine.elaissaoui@nxp.com>