net: dsa: mv88e6xxx: factorize PHY indirect access

Some switch has dedicated SMI PHY Command and Data registers, used to
indirectly access the PHYs, instead of direct access.

Identify these switch models and make mv88e6xxx_phy_{read,write} generic
enough to support every models.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>