igep00x0: enable CONFIG_NET_RANDOM_ETHADDR

Enable CONFIG_NET_RANDOM_ETHADDR to generate a random MAC address
when ETHADDR is not set.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>