Document support for clock-frequency and add details on why this
property is needed and what values are supported.

From internal documentation, while other values are supported, the
correct function of the MDIO bus is not assured hence add only the
suggested supported values to the property enum.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>