PM / Domains: Add a detach callback to the struct dev_pm_domain

The intent of this callback is to simplify detachment of devices from
their PM domains. Further patches will show the benefit.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>