dt-bindings: add binding for rk3366 power domains

Add binding documentation for the power domains
found on Rockchip RK3366 SoCs.

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