PM / Domains: Don't return -EEXIST at attach when PM domain exists

As dev_pm_domain_attach() isn't the only way to assign PM domain pointers
to devices, clearly we must allow a device to have the pointer already
being assigned. For this reason, return 0 instead of -EEXIST.

Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>