dt-bindings: modify document of Rockchip power domains

Modify binding documentation for the power domains
found on Rockchip RK3368 SoCs.

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