boards: nordic: Add sysbuild for NS board (nrf54l10)

-This commit adds "sysbuild: true" for ns build-targets for the
 following nordic board:
  - nrf54l15dk (nrf54l10 target)
-Added missing vendor: nordic for some boards for consistency

Signed-off-by: Frank Audun Kvamtrø <frank.kvamtro@nordicsemi.no>