ACPI dock: support multiple ACPI dock devices

There may be multiple ACPI dock devices exist in ACPI namespace
and we should probe all of them.
http://bugzilla.kernel.org/show_bug.cgi?id=15521



CC: Li Shaohua <shaohua.li@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>