net: davinci_mdio: gaurd the DT code with IS_ENABLED(CONFIG_OF)

guard the davinci_mdio_of_mtable table and davinci_mdio_probe_dt()
with CONFIG_OF.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>