dt-bindings: mailbox: Add a binding for the sun6i msgbox

This mailbox hardware is present in Allwinner sun6i, sun8i, sun9i, and
sun50i SoCs. Add a device tree binding for it.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>