binding: Make "mdio-parent-bus" property from mandatory to optional

Change "mdio-parent-bus" from mandatory section to optional
as it won't be required by integrated MDIO multiplexer
which has bus selection and mdio transaction generation logic,
integrated inside.
Signed-off-by: NPramod Kumar <pramod.kumar@broadcom.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mdio-mux.txt

@@ -5,11 +5,12 @@ numbered uniquely in a device dependent manner.  The nodes for an MDIO
 bus multiplexer/switch will have one child node for each child bus.
 
 Required properties:
-- mdio-parent-bus : phandle to the parent MDIO bus.
 - #address-cells = <1>;
 - #size-cells = <0>;
 
 Optional properties:
+- mdio-parent-bus : phandle to the parent MDIO bus.
+
 - Other properties specific to the multiplexer/switch hardware.
 
 Required properties for child nodes: