boards: silabs: slstk3401a: Fix unit and first address mismatch

This fixes the following warning:

> unit address and first address in 'reg' (0x3e800) don't match for
> /soc/flash-controller@400e0000/flash@0/partitions/partition@fe800

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