dt-bindings: add bindings for rk3399 clock controller

Add devicetree bindings for Rockchip cru which found on
Rockchip SoCs.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>