mem-cache: Fix PIF prefetcher compilation error with NULL ISA

Referencing BaseCPU is causing a compilation error when using the NULL ISA.
This patch changes the reference to a SimObject, which fixes the problem.

Change-Id: I2530486cab65974f5b83e54a733c4b0e98730d26
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17731
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

src/mem/cache/prefetch/Prefetcher.py

@@ -43,7 +43,6 @@ from m5.SimObject import *
 from m5.params import *
 from m5.proxy import *
 
-from m5.objects.BaseCPU import BaseCPU
 from m5.objects.ClockedObject import ClockedObject
 from m5.objects.IndexingPolicies import *
 from m5.objects.ReplacementPolicies import *
@@ -481,6 +480,6 @@ class PIFPrefetcher(QueuedPrefetcher):
         "Replacement policy of the index")
 
     def listenFromProbeRetiredInstructions(self, simObj):
-        if not isinstance(simObj, BaseCPU):
-            raise TypeError("argument must be of BaseCPU type")
+        if not isinstance(simObj, SimObject):
+            raise TypeError("argument must be of SimObject type")
         self.addEvent(HWPProbeEventRetiredInsts(self, simObj,"RetiredInstsPC"))