scripts: runners: nrf: Fix flashing of FLPR/PPR on nRF54H20

After commit aaefaad8, flashing of the FLPR and PPR cores in the nRF54H20
was broken due to those cores missing from the _get_core() function that
retrieves the core to program, triggering an exception. Fix it by
referencing those cores and mapping them to the application core.

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