Documentation: update Ethernet PHY devices binding with 'max-speed'

The 'max-speed' property is optional but defined in the ePAPR
specification and now supported by the Linux Device Tree parsing
infrastructure.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/phy.txt

@@ -22,6 +22,7 @@ Optional Properties:
   specifications. If neither of these are specified, the default is to
   assume clause 22. The compatible list may also contain other
   elements.
+- max-speed: Maximum PHY supported speed (10, 100, 1000...)
 
 Example: