dt-bindings: adc: add description for rv1108 saradc

Add device tree bindings document for saradc on
rockchip rv1108 soc.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>