dev-arm: A SystemCounterListener doesn't have to be Serializable

dev-arm: A SystemCounterListener doesn't have to be Serializable

The class is not making use of any Serializable utility.
By removing this dependency we can extend it more easilly

Change-Id: Ia321b8f0deeb92adde008551eb921dcfd365e675
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39698
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>