defconfig: arm64: Enable USB Link test and network drivers on sa8155

Enable the Link Layer Test driver to allow for SuperSpeed link
layer testing. Also enable Multi-purpose USB Networking Framework
to support USB ethernet adapters.

Change-Id: I47c7adfe60b13b57af6c9d79a872f1ece3331b5f
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>