dev: Use the new Port role mechanism to make an EtherInt Port type.

dev: Use the new Port role mechanism to make an EtherInt Port type.

This type always uses the same "ETHERNET" role, and all connections are
symmetric and self compatible.

Change-Id: I1d060f8c098d52ae9f65c052f816bc813590ff9c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18171
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>