IB/core: Add definition for the standard RoCE V2 UDP port

This will be used in hardware device driver when building QP or AH
contexts.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>