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>