Convert the rockchip,rk3368-mailbox binding to DT schema format. Add
the missing 'clocks' and 'clock-names' properties. Document that it's
one interrupt per mailbox channel (and there are 4 channels).
Link: https://lore.kernel.org/r/20250812181348.62137-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
