ACPI / PM: Assign the ->detach() callback when attaching the PM domain

ACPI / PM: Assign the ->detach() callback when attaching the PM domain

As as preparation to simplify the detachment of devices from their PM
domains, we assign the ->detach() callback to genpd_dev_pm_detach().

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>
(cherry picked from commit 86f1e15f5646b4855bd77025c950239650c4843e)
(cherry picked from commit 5856005b1a1f4aecef050de96a02e1c757049fa0)