boards: nordic: nRF54L15DK: Add debug configuration for FLPR

Add the required code for `west debug` to work the FLPR
core over JLink in the nRF54L 05, 10 and 15 devices.

Note that this requries an external J-Link probe, it will not work with
the on-board (OB) probe soldered on the DK.

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