net: korina: Get mdio input clock via common clock framework

With device tree clock is provided via CCF. For non device tree
use a maximum clock value to not overclock the PHY. The non device
tree usage will go away after platform is converted to DT.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: David S. Miller <davem@davemloft.net>