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>

Author: frkv

boards/nordic/nrf54l15dk/nrf54l15dk_nrf54l10_cpuapp_ns.yaml

@@ -19,3 +19,5 @@ supported:
   - watchdog
   - adc
   - i2s
+vendor: nordic
+sysbuild: true