ACPI: dock: add access to ACPI docking station UID

It is useful to know whether your laptop is docked or not,
but it is even more useful to know which docking station it's
docked to. Attached patch adds "uid" file to sysfs.
Tested on Dell Latitude D600 with D/Dock.
Patch is against official 2.6.20 release.

Signed-off-by: Len Brown <len.brown@intel.com>