net: phy: dp83867: Add documentation for SGMII mode type

Add documentation of ti,sgmii-ref-clock-output-enable
which can be used to select SGMII mode type (4 or 6-wire).

Signed-off-by: Vitaly Gaiduk <vitaly.gaiduk@cloudbear.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ti,dp83867.txt

@@ -37,6 +37,10 @@ Optional property:
 			      for applicable values.  The CLK_OUT pin can also
 			      be disabled by this property.  When omitted, the
 			      PHY's default will be left as is.
+	- ti,sgmii-ref-clock-output-enable - This denotes which
+				    SGMII configuration is used (4 or 6-wire modes).
+				    Some MACs work with differential SGMII clock.
+				    See data manual for details.
 
 Note: ti,min-output-impedance and ti,max-output-impedance are mutually
       exclusive. When both properties are present ti,max-output-impedance