dt-bindings: add documentation for rk3126 clock

This add bindings documentation for rk3126 SoCs.
Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/clock/rockchip,rk3128-cru.txt

-* 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.