IB/rdmavt: Add process MAD stub

This adds the stub for process mad. More study is needed to determine the
final MAD interaction between the driver and rvt.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>