dt-bindings: describe the support of "clock-frequency" in mdio

mdio bus frequency is going to be configurable at boottime by a property
in DT now, so add a description to it.

Signed-off-by: Andy Chiu <[e-mail]>
Reviewed-by: Greentime Hu <[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/xilinx_axienet.txt

···
  - mdio		: Child node for MDIO bus. Must be defined if PHY access is
 		  required through the core's MDIO interface (i.e. always,
 		  unless the PHY is accessed through a different bus).
+		  Non-standard MDIO bus frequency is supported via
+		  "clock-frequency", see mdio.yaml.
 
  - pcs-handle: 	  Phandle to the internal PCS/PMA PHY in SGMII or 1000Base-X
 		  modes, where "pcs-handle" should be used to point