ARM: dts: meson8m2: use the Meson8m2 specific SAR ADC compatible

The SAR ADC on Meson8m2 is slightly different compared to Meson8. The
ADC functionality is identical but the calibration of the internal
thermal sensor is different.

Use the Meson8m2 specific compatible so the temperature sensor is
calibrated correctly on boards using the Meson8m2 SoC.

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