dt-bindings: add documentation of rk3228 clock controller

Add the devicetree binding for the cru on the rk3228 which quite similar
structured as previous clock controllers.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>