net: keystone_net: increase MDIO clock frequency

With MAC_PHY sgmii configuration, u-boot checks PHY link status before
sending each packet. Increasing MDIO frequency increases overall tftp
speed. We set it to maximum 2.5MHz.

Acked-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>