Add description for rk3568 saradc. Signed-off-by: Simon Xue <xxm@rock-chips.com> Reviewed-by: Heiko Stuebner <heiko@sntech.de> Acked-by: Rob Herring <robh@kernel.org> Link: https://lore.kernel.org/r/20210712014437.97427-1-xxm@rock-chips.com Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>