dt-bindings: add document of Rockchip power domains

This add the necessary binding documentation for the power domains
found on Rockchip SoCs.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>