ARM: dts: sunxi: a83t: Add msgbox node

The A83T 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.

Tested-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>