net: phy: marvell: enable downshift by default

A number of PHYs support the PHY tunable to set and get
downshift. However, only 88E1116R enables downshift by default. Extend
this default enabled to all the PHYs that support the downshift
tunable.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>