ACPICA: DeRefOf operator: Update to fully resolve FieldUnit and BufferField refs.

Previously, references to these objects were resolved only to the actual
FieldUnit or BufferField object. The correct behavior is to resolve these
references to an actual value.
The problem is that DerefOf did not resolve these objects to actual
values.  An "Integer" object is simple, return the value.  But a field in
an operation region will require a read operation.  For a BufferField, the
appropriate data must be extracted from the parent buffer.

NOTE: It appears that this issues is present in Windows7 but not
Windows8.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>