sandbox: add dummy driver ETH_SANDBOX_LWIP

sandbox: add dummy driver ETH_SANDBOX_LWIP

Introduce ETH_SANDBOX_LWIP which enables a mock driver similar to
ETH_SANDOX but without the dependencies on the legacy network stack
(NET) so that it may be enabled when the lwIP stack (NET_LWIP) is
introduced. The driver does nothing at this stage but its presence
will allow dm_test_iommu_noiommu [1] to pass.

[1] ./u-boot -T -c "ut dm dm_test_iommu_noiommu"

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>