drivers: eth: stellaris: Enable automatic Ethernet support in QEMU

When used suitable config overlay, qemu_cortex_m3 with Ethernet
support can be started with just usual "make run".

An example of such overlay is included with samples/net/echo_server,
can be built and run with:

make BOARD=qemu_cortex_m3 \
    CONF_FILE="prj.conf overlay-qemu_cortex_m3_eth.conf" run

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>