dt-bindings: rockchip-mailbox: Add mailbox controller document on Rockchip SoCs

This add the necessary binding documentation for mailbox
found on RK3368 SoC.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>