dt-bindings: clk: add rockchip,grf property for RK3399

Add support for rockchip,grf property which is used for GRF muxes
on RK3399.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/clock/rockchip,rk3399-cru.txt

@@ -13,6 +13,12 @@ Required Properties:
 - #clock-cells: should be 1.
 - #reset-cells: should be 1.
 
+Optional Properties:
+
+- rockchip,grf: phandle to the syscon managing the "general register files".
+  It is used for GRF muxes, if missing any muxes present in the GRF will not
+  be available.
+
 Each clock is assigned an identifier and client nodes can use this identifier
 to specify the clock which they consume. All available clocks are defined as
 preprocessor macros in the dt-bindings/clock/rk3399-cru.h headers and can be