dt-bindings: iio: adc: convert rockchip saradc bindings to yaml

Current dts files with 'saradc' nodes are manually verified.
In order to automate this process rockchip-saradc.txt
has to be converted to yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>