soc: Allow test-building of MediaTek drivers

The COMPILE_TEST alternative dependency allows test-building the
drivers but only as long as the build system itself will look into
the directory where the drivers reside.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>