dt-bindings: adc: add description for rv1108 saradc

Add device tree bindings document for saradc on
rockchip rv1108 soc.
Signed-off-by: NAndy Yan <andy.yan@rock-chips.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -6,6 +6,7 @@ Required properties:
    - "rockchip,rk3066-tsadc": for rk3036
    - "rockchip,rk3328-saradc", "rockchip,rk3399-saradc": for rk3328
    - "rockchip,rk3399-saradc": for rk3399
+   - "rockchip,rv1108-saradc", "rockchip,rk3399-saradc": for rv1108
 
 - reg: physical base address of the controller and length of memory mapped
        region.