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>

Documentation/devicetree/bindings/net/xilinx_axienet.txt

@@ -37,6 +37,9 @@ Optional properties:
 		  auto-detected from the CPU clock (but only on platforms where
 		  this is possible). New device trees should specify this - the
 		  auto detection is only for backward compatibility.
+ - mdio		: Child node for MDIO bus. Must be defined if PHY access is
+		  required through the core's MDIO interface (i.e. always,
+		  unless the PHY is accessed through a different bus).
 
 Example:
 	axi_ethernet_eth: ethernet@40c00000 {