iio: mxs-lradc: compute temperature from channel 8 and 9

The mxs LRADC is able to read an internal die temperature sensor. The
temperature has to be calculated from the value read on channel 8 and channel 9.
To be able to expose the result to hwmon, implement iio channel 8 as
(channel 9 - channel 8). Then, implement IIO_CHAN_INFO_SCALE and
IIO_CHAN_INFO_OFFSET so that it can be processed by hwmon through the in kernel
provider/consumer mechanism.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
ARM: dts: mxs: add #io-channel-cells to mx28 lradc

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
ARM: dts: mxs: Add iio-hwmon to mx28 boards

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>