ASoC: meson: aiu: Document Meson8 and Meson8b support in the dt-bindings

The AIU audio output controller on the Meson8 and Meson8b SoC families
is compatible with the one found in the GXBB family. Document the
compatible string for these two older SoCs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200220205711.77953-2-martin.blumenstingl@googlemail.com
Signed-off-by: Mark Brown <broonie@kernel.org>