ACPI: Add _UID support for ACPI devices.

The _UID object is optional, but is required when the device has no
other way to report a persistent unique device ID.
This patch is required for ACPI 5.0 ACPI enumerated IP cores.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rui Zhang <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>