arch: Add generic BaseMMU

arch: Add generic BaseMMU

This is an abstract class encapsulating the ITB and DTB
(Instruction and Data TLBs)

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

Change-Id: I7c8fa2ada319e631564182075da1aaff517ec212
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34975
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>