Visible only if COMPILE_TEST is enabled, this allows to include the
driver in build tests.

Cc: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: NMichal Simek <michal.simek@xilinx.com>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>