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>