python: Always throw TypeError on slave-slave connections

python: Always throw TypeError on slave-slave connections

params.py checks the validity of memory port-port connections before
they are instantiated in C++. This commit ensures that attempting to
connect two slave ports together will cause a TypeError.

Change-Id: Ia7d0a15df28b96c7bf5e568c4f4917d21a19b824
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15896
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>