net: mvpp2: initialize port of_node pointer

Without a valid of_node in struct device we can't find the mvpp2 port
device by its DT node. Specifically, this breaks
of_find_net_device_by_node().

For example, the Armada 8040 based Clearfog GT-8K uses Marvell 88E6141
switch connected to the &cp1_eth2 port:

&cp1_mdio {
	...

	switch0: switch0@4 {
		compatible = "marvell,mv88e6085";
		...

		ports {
			...

			port@5 {
				reg = <5>;
				label = "cpu";
				ethernet = <&cp1_eth2>;
			};
		};
	};
};

Without this patch, dsa_register_switch() returns -EPROBE_DEFER because
of_find_net_device_by_node() can't find the device_node of the &cp1_eth2
device.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>