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>