samples: drivers: mbox: Fix overlays for nrf54h20

Add sample configuration 'nrf54h20_app_ppr' where:
- cpuapp sends messages to cpuppr vevif channel 15,
- cpuppr sends messages to cpuapp bellboard channel 18.

Add sample configuration 'nrf54h20_rad_app' where:
- cpuapp sends messages to cpurad bellboard channel 12,
- cpurad sends messages to cpuapp bellboard channel 18.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>