dt-bindings: add documentation for rk3126 clock

* Rockchip RK3128 Clock and Reset Unit

* Rockchip RK3126/RK3128 Clock and Reset Unit

The RK3128 clock controller generates and supplies clock to various

The RK3126/RK3128 clock controller generates and supplies clock to various

controllers within the SoC and also implements a reset controller for SoC

peripherals.

Required Properties:

- compatible: should be "rockchip,rk3128-cru"

- compatible: should be "rockchip,rk3126-cru" or "rockchip,rk3128-cru"

  "rockchip,rk3126-cru" - controller compatible with RK3126 SoC.

  "rockchip,rk3128-cru" - controller compatible with RK3128 SoC.

- reg: physical base address of the controller and length of memory mapped

  region.

- #clock-cells: should be 1.