net: dsa: mv88e6xxx: Add eeprom-length to binding

The binding documentation for the mv88e6xxx switch is missing the
eeprom-length property, which has been implemented since May 2016,
commit f8cd8753 ("dsa: mv88e6xxx: Handle eeprom-length property")

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