dt-bindings: net: phy: mscc: vsc8531: Add LED mode combine disable property

Add the new property to disable the combined LED modes to the bindings
documentation.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>

Documentation/devicetree/bindings/net/mscc-phy-vsc8531.txt

@@ -31,6 +31,8 @@ Optional properties:
 			  VSC8531_LINK_100_ACTIVITY (2),
 			  VSC8531_LINK_ACTIVITY (0) and
 			  VSC8531_DUPLEX_COLLISION (8).
+- vsc8531,led-[N]-combine-disable	: Disable the combined mode for LED[N].
+			  This disables the second mode if a combined mode is selected.
 - load-save-gpios	: GPIO used for the load/save operation of the PTP
 			  hardware clock (PHC).