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

Cc: Damien Horsley <Damien.Horsley@imgtec.com>
Acked-by: NJames Hartley <james.hartley@imgtec.com>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>