dt-bindings: arm: add the Meson8m2 Tronsmart MXIII

The MXIII doesn't contain an actual Tronsmart label on the case.
However, the PCB is almost identical (button setup, PMU, USB
connections, ...). The main differences between the MXIII and MXIII-Plus
are:
- Ampak AP6330 on the MXIII-Plus, Ampak AP6181 on the MXIII
- Realtek RTL8211F on the MXIII-Plus, IC Plus IP101A/G on the MXIII
- Netxeon markings on the PCB on the MXIII-Plus, nothing on the MXII
- Tronsmart writing on the top of the case of the MXIII-Plus, "MXIII"
  writing (but no Tronsmart) on the MXIII

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>