configs: set CONFIG_NET=y for FTGMAC100

FTGMAC100 is not compatible with NET_LWIP which was enabled as the
default stack in a previous commit. So enable NET in the defconfig.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>