arm64: dts: allwinner: a64: Add msgbox node

The A64 SoC contains 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>