dt-bindings: iio: adc: rockchip-saradc: add description for px30

The description below is already in use for px30.dtsi,
but was somehow never added to a document, so add
"rockchip,px30-saradc", "rockchip,rk3399-saradc"
for saradc nodes on a px30 platform to rockchip-saradc.yaml.

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