cpu, probe: Fix elastic trace register dependency

Change-Id: I017852eac183fac3f914fdb96d7e72a56ea9d682
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5121
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>