net: airoha: Add loopback support for GDM2

Enable hw redirection for traffic received on GDM2 port to GDM{3,4}.
This is required to apply Qdisc offloading (HTB or ETS) for traffic to
and from GDM{3,4} port.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>