Staging/iio: add TODO reminder

Some things have still to be done to the LRADC driver.
Signed-off-by: NJuergen Beisert <jbe@pengutronix.de>
Acked-by: NAcked-by: Marek Vasut <marex@denx.de>
Signed-off-by: NJonathan 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>

drivers/staging/iio/TODO

@@ -13,6 +13,17 @@ Would be nice
 3) Expand device set. Lots of other maxim adc's have very
    similar interfaces.
 
+MXS LRADC driver:
+This is a classic MFD device as it combines the following subdevices
+ - touchscreen controller (input subsystem related device)
+ - general purpose ADC channels
+ - battery voltage monitor (power subsystem related device)
+ - die temperature monitor (thermal management)
+
+At least the battery voltage and die temperature feature is required in-kernel
+by a driver of the SoC's battery charging unit to avoid any damage to the
+silicon and the battery.
+
 TSL2561
 Would be nice
 1) Open question of userspace vs kernel space balance when