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>

src/arch/arm/ArmPMU.py

@@ -115,8 +115,9 @@ class ArmPMU(SimObject):
         sharing the PMU (e.g., when switching between CPU models).
         """
 
-        bpred = cpu.branchPred if cpu and not isNullPointer(cpu.branchPred) \
-            else None
+        bpred = getattr(cpu, "branchPred", None) if cpu else None
+        if bpred is not None and isNullPointer(bpred):
+            bpred = None
 
         self.addEvent(SoftwareIncrement(self,0x00))
         # 0x01: L1I_CACHE_REFILL