ACPI: Add user space interface for identification objects

ACPI devices are glued with physical devices through _ADR object, ACPI
enumerated devices are identified with _UID object.  Currently we can
observe _HID/_CID through sysfs interfaces (hid/modalias), but there's
no way for us to check _ADR/_UID from user space.  This patch closes
this gap for ACPI developers and users.

[rjw: Modified the subject and changelog slightly.]
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>