boards: nordic: nrf52805: Fix slot1_partition base address

This prevents an overlap with `storage_partition`

Additionally, it fixes the following warning:

> unit address and first address in 'reg' (0x20000) don't match for
> /soc/flash-controller@4001e000/flash@0/partitions/partition@19000

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