boards: nordic: Add sysbuild for NS boards

-This commit adds "sysbuild: true" for ns build-targets for the
 following nordic boards:
  - nrf5340_audio_dk
  - nrf54l15dk
  - nrf9131ek
  - nrf9151dk
  - nrf9161dks (multiple versions)
  - thingy53
-Added missing vendor: nordic for some boards for consistency

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