dt-bindings: iio: adc: document the Meson G12A support

Update the documentation to explicitly support the Meson-G12A SoC.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>