dt-bindings: net: phy: smsc: document reference clock

Add support to specify the reference clock for the phy.
Signed-off-by: NMarco Felsch <m.felsch@pengutronix.de>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/smsc-lan87xx.txt

@@ -5,6 +5,10 @@ through an Ethernet OF device node.
 
 Optional properties:
 
+- clocks:
+  The clock used as phy reference clock and is connected to phy
+  pin XTAL1/CLKIN.
+
 - smsc,disable-energy-detect:
   If set, do not enable energy detect mode for the SMSC phy.
   default: enable energy detect mode