boards: nordic: nrf54l15dk: Set the right --device flag for JLink

Starting in version 8.10f, Segger's JLink now supports the nRF54L15's
Cortex-M33 core natively, being able to flash it and debug it properly.
Use this new support in the Zephyr built-in board support.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>