net: dsa: mv88e6xxx: factorize EEPROM access

Add a MV88E6XXX_FLAG_EEPROM flag to describe switch models featuring an
EEPROM and distribute the EEPROM access routines to all models.

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