cpu, dev, mem: Use the new Port methods.

cpu, dev, mem: Use the new Port methods.

Use getPeer, takeOverFrom, and << to simplify the use of ports in some
areas.

Change-Id: Idfbda27411b5d6b742f5e4927894302ea6d6a53d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20235
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>