net: mvmdio: add clocks property to binding documentation

Commit 3d604da1
  ("net: mvmdio: get and enable optional clock")

was missing an update of the corresponding device tree binding
documentation. This patch adds the clocks property to mvmdio
binding documentation.
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/marvell-orion-mdio.txt

@@ -11,6 +11,7 @@ Required properties:
 
 Optional properties:
 - interrupts: interrupt line number for the SMI error/done interrupt
+- clocks: Phandle to the clock control device and gate bit
 
 The child nodes of the MDIO driver are the individual PHY devices
 connected to this MDIO bus. They must have a "reg" property given the