ARM: dts: meson8m2: mxiii-plus: add iio-hwmon for the chip temperature

SAR ADC enabled channel 8 can be used to measure the chip temperature.
This can be made available to the hwmon subsystem by using iio-hwmon.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>