A.T. Robertson's Word Pictures also favors the understanding "faithfulness."
Vincent's Word Studies:
Alford GNT: "πίστις, in the widest sense: faith, towards God and man: of love it is said, 1 Corinthians 13:7, πάντα πιστεύει."
NET Bible renders πίστις as "faithfulness," but in a note for Gal 5:22, it adds:
Or "reliability"; see BDAG 818 s.v. πίστις 1.a.