net-next: ethernet: mediatek: add device_node of GMAC pointing into the netdev instance

the patch adds the setup of the corresponding device node of GMAC into the
netdev instance which could allow other modules such as DSA to find the
instance through the node in dt-bindings using of_find_net_device_by_node()
call.

Signed-off-by: Sean Wang <sean.wang@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>