net: phy: dp83867: Add documentation for CLK_OUT pin muxing

Add documentation of ti,clk-output-sel which can be used to select
a specific clock for CLK_OUT.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -25,6 +25,8 @@ Optional property:
 				    software needs to take when this pin is
 				    strapped in these modes. See data manual
 				    for details.
+	- ti,clk-output-sel - Muxing option for CLK_OUT pin - see dt-bindings/net/ti-dp83867.h
+				    for applicable values.
 
 Note: ti,min-output-impedance and ti,max-output-impedance are mutually
       exclusive. When both properties are present ti,max-output-impedance