net: stmmac: dwmac-oxnas: fix of-node leak

Use the syscon lookup-by-phandle helper so that the reference taken by
of_parse_phandle() is released when done with the node.

Fixes: 5ed74140

 ("net: stmmac: Add OXNAS Glue Driver")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>