dt-bindings: add binding for rk3399 power domains

Add binding documentation for the power domains
found on Rockchip RK3399 SoCs

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>