dt-bindings: add documentation of rk3036 clock controller

Add the devicetree binding for the cru on the rk3036 which quite similar
structured as previous clock controllers.

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