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>
