ACPI: dock: add uevent to indicate change in device status

Send a uevent to indicate a device change whenever we dock or
undock, so that userspace may now check the dock status via sysfs.

Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Holger Macht <hmacht@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>