Add support for rockchip,grf property which is used for GRF muxes
on RK3399.
Signed-off-by: NXing Zheng <zhengxing@rock-chips.com>
Reviewed-by: NDouglas Anderson <dianders@chromium.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>