boards: seagate: legend: Fix unit and first address mismatch

This fixes the following warning:

> unit address and first address in 'reg' (0x1000) don't match for
> /soc/spi@40003800/spi_nor@0/partitions/partition@10000

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