dt-bindings: iio: adc: Add docs for ad7124

Add support for Analog Devices AD7124 4-channels and 8-channels ADC.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>