boards: nxp: usb_kw24d512: Fix unit and first address mismatch

This fixes the following warning:

> unit address and first address in 'reg' (0x70000) don't match for
> /soc/flash-controller@40020000/flash@0/partitions/partition@700000

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