arm: Demote PCEvent subclass pointers to PCEvent pointers.

arm: Demote PCEvent subclass pointers to PCEvent pointers.

Nothing is actually accessed through these pointers. This simplifies
their declration, and gives more flexibility when setting up those
events.

Change-Id: If857de5c8df37b6ead7eae53e3c0c6c3103938c0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24112
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>