net: phy: marvell: add downshift support for 88E1145

Add downshift support for 88E1145, it uses the same downshift
configuration registers as 88E1111.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>