samples: drivers: mbox: Add mbox IRQ configuration

The mbox sample uses bellboard events enabled by default
in the nRF54H20dk board definition.
Thus, it's not easily obvious that IRQ bitmask must be aligned
when other mbox/bellboard channel is used.

Add comment to the nRF54H20 overlays.
Inform that bellboard channel used for RX must be enabled in
the 'nordic,interrupt-mapping' property.

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