iio: light: add missing vcnl4040 of_compatible

Commit 5a441aad ("iio: light: vcnl4000 add support for the VCNL4040
proximity and light sensor") added the support for the vcnl4040 but
forgot to add the of_compatible. Fix this by adding it now.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Fixes: 5a441aad ("iio: light: vcnl4000 add support for the VCNL4040 proximity and light sensor")
Reviewed-by: Angus Ainslie (Purism) <angus@akkea.ca>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>