net: dsa: mv88e6xxx: abstract PHY ops

Old chips use a direct access to the PHY devices registers. Next chips
have a PHY Polling Unit (PPU) which needs to be disabled before
accessing PHY registers. Newer chips have an indirect access to the PHY
devices so that disabling the PPU is not necessary.

Introduce a new phy_ops structure in the chip to describe the required
PHY access routines.

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