dt-bindings: add bindings for Rockchip grf

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

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>