arch-arm: Add missing fall-through defaults

Change-Id: Ie64b83d754c4719a77c7788879be71304a9b786e
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17289
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andrea Mondelli <Andrea.Mondelli@ucf.edu>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>