IB/rdmavt: Add the start of capability flags

Drivers will need a set of flags to dictate behavior to rdmavt. This patch
adds a placeholder and a spot for it to live, as well as a few flags
that will be used.

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