dt-bindings: iio: adc: add Meson8 and Meson8b support

The Amlogic Meson SAR ADC driver can be used on Meson8 and Meson8b
(probably on earlier SoC generations as well, but I don't have any
hardware available for testing that).
Add a separate compatible for Meson8 and Meson8b because it does not
need any of the BL30 magic (unlike the GX SoCs).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>