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: Pramod Kumar <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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

···
 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: