ACPICA: Add support to externally execute _OSI method

The current implemenation of _OSI within ACPICA only allows other
control methods to execute _OSI. This change allows the host
OS to execute _OSI via the AcpiEvaluateObject interface. _OSI
is a special method -- it does not exist in the AML code, it is
implemented within ACPICA.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>