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: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NBaruch Siach <baruch@tkos.co.il>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c

@@ -4803,6 +4803,7 @@ static int mvpp2_port_probe(struct platform_device *pdev,
 	dev->min_mtu = ETH_MIN_MTU;
 	/* 9704 == 9728 - 20 and rounding to 8 */
 	dev->max_mtu = MVPP2_BM_JUMBO_PKT_SIZE;
+	dev->dev.of_node = port_node;
 
 	/* Phylink isn't used w/ ACPI as of now */
 	if (port_node) {