iio: adc: meson-saradc: add support for the Meson8m2 SoCs

The SAR ADC on Meson8m2 behaves identical to the one found in the
Meson8b SoCs. Add a separate compatible string because the temperature
sensor logic (not supported yet) differs between Meson8 and Meson8m2
(however, it's the same for Meson8b and Meson8m2).

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