misc: BaseCPU using ArchMMU instead of ArchDTB/ArchITB

misc: BaseCPU using ArchMMU instead of ArchDTB/ArchITB

With this commit we replace every TLB pointer stored in the
cpu model with a BaseMMU pointer.

JIRA: https://gem5.atlassian.net/browse/GEM5-790

Change-Id: I4932a32f68582b25cd252b5420b54d6a40ee15b8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34976
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>