board: atmel: common: introduce at91_set_eth1addr for second interface

We already have a function to retrieve the mac address from one EEPROM.
For boards with a second Ethernet interface, however, we would
require another EEPROM with a second unique MAC address.
Introduce at91_set_eth1addr which will look for a second EEPROM
and set the 'eth1addr' variable with the obtained MAC address.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>