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>