dt-bindings:iio:adc: add sprd,ump9620-adc dt-binding

sprd,ump9620-adc is one variant of sc27xx series, add ump9620
description and sample in dt-bindings.

Signed-off-by: Cixi Geng <cixi.geng1@unisoc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220419142458.884933-2-gengcixi@gmail.com


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