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>