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

UPSTREAM: 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.

Change-Id: Ie2a79aab6e1e073157c06d44252ef327caee5261
Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
[restructured error handling in subdomain-addition]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
(cherry picked from git://git.kernel.org/pub/scm/linux/kernel/git/mmind/linux-rockchip.git
 v4.7-armsoc/drivers commit 6be05b5ec16132f3df3f1d857ab01e30f726b542)