net: phy: mscc-miim: add support to set MDIO bus frequency

Until now, the MDIO bus will have the hardware default bus frequency.
Read the desired frequency of the bus from the device tree and configure
it.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>