IB/rdmavt: Add pkey query stub

The pkey table will reside in the rvt structure but it will be modified
only when the driver requests then rvt will simply read the value to return
in the query.

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>