dt-bindings: iio: adc: rockchip-saradc: add description for rk3568

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>