net: phy: marvell: add downshift support for M88E1111

This patch adds downshift support for M88E1111. This PHY version uses
another register for downshift configuration, reading downshift status
is possible via the same register as for other PHY versions.

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>