dt-bindings: iio: adc: Add Xilinx AMS binding documentation

Xilinx AMS have several ADC channels that can be used for measurement of
different voltages and temperatures. Document the same in the bindings.

Signed-off-by: Anand Ashok Dumbre <anand.ashok.dumbre@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211203212358.31444-5-anand.ashok.dumbre@xilinx.com


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