net: axienet: document device tree mdio child node

The mdio child node for the MDIO bus is generally required when using
this driver but was not documented other than being shown in the
example. Document it as an optional (but usually required) parameter.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>