net-next: ethernet: mediatek: add CDM able to recognize the tag for DSA

The patch adds the setup for allowing CDM can recognize these packets with
carrying port-distinguishing tag. Otherwise, these tagging packets will be
handled incorrectly by CDM. The setup is also working out for general
untag packets as well.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Landen Chao <Landen.Chao@mediatek.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>