dsa: mv88e6xxx: Handle eeprom-length property

A switch can export an attached EEPROM using the standard ethtool API.
However the switch itself cannot determine the size of the EEPROM, and
multiple sizes are allowed. Thus a device tree property is supported
to indicate the length of the EEPROM. Parse this property during
device probe, and implement a callback function to retrieve it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>