soc: rockchip: power-domain: add support for sub-power domains

This patch adds support for making one power domain a sub-domain of
other domain. This is useful for modeling power dependences,
which needs to have more than one power domain enabled to be operational.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
[restructured error handling in subdomain-addition]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>