arch-generic: Making base TLB class a MemObject

Allow configuring a TLB hierarchy using ports

Change-Id: I1f791829d4e072a9104e67eacf69a69de9543634
Reviewed-on: https://gem5-review.googlesource.com/c/14117
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>