dt-bindings: iio/adc: add an INGENIC_ADC_AUX0 entry

The JZ4760(B) socs have 3 AUX inputs, add an entry to prepare including the one named AUX in the sadc driver.
Leaving the rest untouched as it's ABI.

Signed-off-by: Christophe Branchereau <cbranchereau@gmail.com>

Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210726082033.351533-3-cbranchereau@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>