boards: renesas: da1469x: Fix unit and first address mismatch

This fixes the following warning:

> unit address and first address in 'reg' (0x82000) don't match for
> /soc/flash-controller@38000000/flash@16000000/partitions/partition@80000

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