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

Update the documentation to expicitly support the Meson-AXG SoC.

Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>