dsa: Add new optional devicetree property to describe EEPROM size

The dsa core now supports reading from and writing to a switch EEPROM
if connected. Describe optional devicetree property indicating that
an EEPROM is present and its size.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>