dt-bindings: arm: amlogic: add support for the Tronsmart MXIII Plus

Add the binding documentation for the Tronsmart MXIII Plus which is an
Android TV box using the Amlogic S812 (Meson8m2) SoC.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>