dt-bindings: net: xgmac_mdio: Remove unsupported "bus-frequency"

This property has never been supported by the driver. The kernel has
settled on "clock-frequency" as the standard name for this binding, so
once that is supported we will document that instead.

Fixes: 7f93c9d9 ("power/fsl: add MDIO dt binding for FMan")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl-fman.txt

@@ -388,15 +388,6 @@ PROPERTIES
 		Value type: <prop-encoded-array>
 		Definition: A standard property.
 
-- bus-frequency
-		Usage: optional
-		Value type: <u32>
-		Definition: Specifies the external MDIO bus clock speed to
-		be used, if different from the standard 2.5 MHz.
-		This may be due to the standard speed being unsupported (e.g.
-		due to a hardware problem), or to advertise that all relevant
-		components in the system support a faster speed.
-
 - interrupts
 		Usage: required for external MDIO
 		Value type: <prop-encoded-array>