dt-bindings: net: lpc-eth: document optional properties

The Ethernet controller is also an mdio controller, to be able to parse
children (phys for example), and mdio node must be present.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/lpc-eth.txt

@@ -10,6 +10,11 @@ Optional properties:
   absent, "rmii" is assumed.
 - use-iram: Use LPC32xx internal SRAM (IRAM) for DMA buffering
 
+Optional subnodes:
+- mdio : specifies the mdio bus, used as a container for phy nodes according to
+  phy.txt in the same directory
+
+
 Example:
 
 	mac: ethernet@31060000 {