dt-bindings: iio: adc: add Meson8m2 support

The Amlogic Meson SAR ADC implementation on the Meson8m2 SoC is
identical to the Meson8b variant. Add a compatible string to indicate
that we support the SAR ADC on the Meson8m2 SoC.

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