boards: nxp: frdm_mcxn236: Fix unit and first address mismatch

This fixes the following warning:

> unit address and first address in 'reg' (0x88000) don't match for
> <snip>/flash-controller@43000/flash@0/partitions/partition@80000

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>