arch-arm: Fix fallthrough when trapping at EL2

arch-arm: Fix fallthrough when trapping at EL2

This had been caused by the introduction of GICv3 registers trapping in
commit 32a23114c14cebc5ec0067ac739144b50e412219

Change-Id: I5073e2891f3ff5c5a9e05d3456dad6f4f8ffba0d
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18909
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>