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

Add description for rk3568 saradc.

Signed-off-by: Simon Xue <[e-mail]>
Reviewed-by: Heiko Stuebner <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/adc/rockchip-saradc.yaml

···
               - rockchip,px30-saradc
               - rockchip,rk3308-saradc
               - rockchip,rk3328-saradc
+              - rockchip,rk3568-saradc
               - rockchip,rv1108-saradc
           - const: rockchip,rk3399-saradc