Joint use agreements. For commercial, office, and industrial centers developed as a single entity, but which include lots under separate ownership, the standards for parking, internal roads, lot coverage and open space may be satisfied by all of the property included within the center. In such event, permanent easements shall be recorded on the approved site plan and subdivision plan, and covenants recorded in the Registry of Deeds providing for the joint use and maintenance of parking, roadways, and open space by all occupants of the center.