arch-arm: Stage1&2 TableWalkers sharing same port

This patch reverts part of the changes made by the removal of
the Stage2MMU class [1]:

Prior to that patch the stage1 and stage2 walkers were sharing
the same port (which was instantiated in the Stage2MMU).
By removing the Stage2MMU we provided every table walker a
unique port.

With this patch we are reintroducing port sharing to temporarily fix
existing platforms using walker caches.
(The long term design goal will be to have a unique page table walker)

Those complain if we try to connect a single ported cache to 2 table
walker ports (stage1 and stage2)

[1]: https://gem5-review.googlesource.com/c/public/gem5/+/45780

Change-Id: Ib68ef97f1e9772a698771269c9a4ec4514f5d4d7
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48200
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed