ACPICA: Add Windows8/Server2012 string for _OSI method.

This change adds a new _OSI string, "Windows 2012" for both Windows 8
and Windows Server 2012.

From Microsoft document "How to Identify the Windows Version in ACPI
by Using _OSI", July 13, 2012.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>