soc: rockchip: power-domain: allow domains only handling idle requests

On some Rockchip SoC there exist child-domains only handling their
idle state with the actual power-state handled by a (shared) parent-
domain.

So allow such types of domains. For them, we can determine their
state (on/off) by checking the inverse idle-state instead.

There exist one special case if both idle as well power handling
were set as not present, but as the domain-data is defined in the
code itself, we can expect the reasonable developer to define them
in a correct way, without adding more checks.

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