ARM: dts: meson8m2: mxiii-plus: create a common .dtsi

There are two "MXIII" boards out there which use an identical board
layout.
Both use a S812 SoC and 1GB or 2GB RAM, depending on the variant. They
also share the same Ricoh RN5T618 PMU, the reset button (behind the AV
port), the power button on top of the case, the blue and red LED (both
are currently not supported yet), two USB A ports (host only), one
micro USB port (OTG), micro SD card slot, an AV connector, a HDMI-A
connector and SPDIF connector.

MXIII-Plus:
- Realtek RTL8211F RGMII Gigabit PHY
- Ampak AP6330 Bluetooth and abgn Wifi combo
- the board contains references to Netxeon
- uses eMMC storage

MXIII:
- IC Plus IP101A/G RMII 10/100 PHY
- Ampak AP6181 bgn Wifi (no Bluetooth)
- no Netxeon references
- uses MLC NAND storage

Prepare a common .dtsi to make it easier to add and maintain support for
the MXIII (no -Plus suffix) variant.

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