ARM: dts: mxs: Add iio-hwmon to mx28 soc

This allows to read the SoC on-die temperature sensor available on the LRADC by
using:
 cat /sys/class/hwmon/hwmon0/device/temp1_input

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>