boards: nxp: frdm_kw41z: Fix unit and first address mismatch

This fixes the following warnings:

> devicetree error: 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>