arch-arm: Don't fail to initialise PMU if BP is missing

The branch predictor currently assumes that each CPU has a branch
predictor. This isn't true in some cases (e.g., KVM, Atomic).

Change-Id: I61e80dca940c2dadba1b812449691c9dba1c06d9
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11520
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>