ARM: configs: at91: sama5: Enable LAN78XX as module

Include this driver as the USB-Ethernet chip is present on SAMA5D2-ICP
board.

Signed-off-by: Razvan Stefanescu <razvan.stefanescu@microchip.com>
[nicolas.ferre@microchip.com: update commit subject and message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>