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

Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Gabriel Fernandez <gabriel.fernandez@st.com>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>