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

Cc: Antoine Tenart <antoine.tenart@free-electrons.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>