ARM: dts: meson: move iio-hwmon for the SoC temperature to meson.dtsi

The SoC temperature can be retrieved from ADC channel 8 on all 32-bit
SoCs (Meson6, Meson8, Meson8b and Meson8m2). Move the iio-hwmon instance
to meson.dtsi instead of duplicating it in all board.dts.
If the temperature sensor calibration data is missing for a board then
iio-hwmon will simply not probe.

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