IB/core: Define 'opa' rdma_ah_attr type

OPA ah_attr types allows core components to specify
attributes that may be specific to opa devices.
For instance, opa type ah_attr provides 32 bit lids
enabling larger OPA fabric sizes.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Dasaratharaman Chandramouli <dasaratharaman.chandramouli@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>