dev: Scrub out some lingering uses of MemObject.

MemObject doesn't do anything any more, and is basically just an alias
for ClockedObject.

Change-Id: Ic0e1658609e4e1d7f4b829fbc421f222e4869dee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20719
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

dev-arm: Implement a SMMUv3 model

This is an implementation of the SMMUv3 architecture.

What can it do?
- Single-stage and nested translation with 4k or 64k granule. 16k would
be straightforward to add.
- Large pages are supported.
- Works with any gem5 device as long as it is issuing packets with a
valid (Sub)StreamId

What it can't do?
- Fragment stage 1 page when the underlying stage 2 page is smaller. S1
page size > S2 page size is not supported
- Invalidations take zero time. This wouldn't be hard to fix.
- Checkpointing is not supported
- Stall/resume for faulting transactions is not supported

Additional contributors:
- Michiel W. van Tol <Michiel.VanTol@arm.com>
- Giacomo Travaglini <giacomo.travaglini@arm.com>

Change-Id: Ibc606fccd9199b2c1ba739c6335c846ffaa4d564
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19008
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>