dt-bindings: iio/adc: Add AUX2 channel idx for JZ4770 SoC ADC

Introduce support for AUX2 channel found in ADC hardware present on
Ingenic JZ4770 SoC.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>