Staging/iio: add TODO reminder

Some things have still to be done to the LRADC driver.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Acked-by: Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
CC: linux-arm-kernel@lists.infradead.org
CC: linux-input@vger.kernel.org
CC: devel@driverdev.osuosl.org
CC: Marek Vasut <marex@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>
CC: Jonathan Cameron <jic23@cam.ac.uk>