net-next: dsa: add Mediatek tag RX/TX handler

Add the support for the 4-bytes tag for DSA port distinguishing inserted
allowing receiving and transmitting the packet via the particular port.
The tag is being added after the source MAC address in the ethernet
header.

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>