sim: Add a takeOverFrom method to the base Port class.

sim: Add a takeOverFrom method to the base Port class.

This makes it easier, safer, and less verbose to swap out ports when
a CPU takes over for another CPU, for instance.

Change-Id: Ice08e4dcb4b04dc66b1841331092a78b4f6f5a96
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20233
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: Gabe Black <gabeblack@google.com>