ARM: dts: sunxi: h3/h5: Add msgbox node

The H3 and H5 SoCs contain a message box that can be used to send
messages and interrupts back and forth between the ARM application CPUs
and the ARISC coprocessor. Add a device tree node for it.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>