cpu: Turn the stage 2 ARM MMUs from params to children.

cpu: Turn the stage 2 ARM MMUs from params to children.

These aren't referred to in the C++, so there's no reason for them to
be parameters. By making them children, they can still be modified,
replaced wholesale, or even replaced by an entirely different object
to, for instance, mask them when they're not needed.

Change-Id: Ic7f144a3cd3d1fca12fec220918aa72af885f61c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21839
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>