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>

boards/silabs/starter_kits/slstk3401a/slstk3401a-common.dtsi

@@ -127,7 +127,7 @@
  #size-cells = <1>;
 
  /* Set 6Kb of storage at the end of the 256Kb of flash */
- storage_partition: partition@fe800 {
+ storage_partition: partition@3e800 {
  label = "storage";
  reg = <0x0003e800 0x00001800>;
  };