net: dsa: mv88e6xxx: add chip-wide ops

Introduce a mv88e6xxx_ops structure to describe supported chip-wide
functions and assign the correct variant to the chip models.

For the moment, add only PHY access routines. This allows to get rid of
the PHY ops structures and the usage of PHY flags.

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