dev: Make EtherInt inherit from Port.

This way a common function can return EtherInt (and master and slave
ports).

Change-Id: I1215baaad918ef0352b228877241b2b6dd2503fc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17035
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>