ARM: dts: meson: add the ADC thermal sensor to meson.dtsi

The SoC temperature can be retrieved from ADC channel 8 on all 32-bit
SoCs (Meson6, Meson8, Meson8b and Meson8m2). Add a "generic-adc-thermal"
instance to meson.dtsi so the thermal sensor is available for all SoCs.
If the temperature sensor calibration data is missing for a board then
the "generic-adc-thermal" will not probe and not register a thermal
sensor.

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