dpaa2-eth: setup the of_node field of the device

When the DPNI object is connected to a DPMAC, setup the of_node to point
to the DTS device node of that specific MAC. This enables other drivers,
for example the DSA subsystem, to find the net_device by its device
node.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>