ACPI / hotplug: Generate online uevents for ACPI containers

Commit 46394fd0 (ACPI / hotplug: Move container-specific code out of
the core) removed the generation of "online" uevents for containers,
because "add" uevents are now generated for them automatically when
container system devices are registered.  However, there are user
space tools that need to be notified when the container and all of
its children have been enumerated, which doesn't happen any more.

For this reason, add a mechanism allowing "online" uevents to be
generated for ACPI containers after enumerating the container along
with all of its children.

Fixes: 46394fd0 (ACPI / hotplug: Move container-specific code out of the core)
Reported-and-tested-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: 3.14+ <stable@vger.kernel.org> # 3.14+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>