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 <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/xilinx_axienet.txt

@@ -68,6 +68,8 @@ Optional properties:
  - 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