kernel: add kmod-iio-htu21

kernel: add kmod-iio-htu21

This adds support for the htu21 humidity and temperature sensor.

To get it to work you have to do something like this:
echo "htu21 0x40" >/sys/class/i2c-dev/i2c-1/device/new_device
for example by adding it to rc.local

Compile tested on brcm2708 and I have used an earlier version of this
patch for more than a year.

Signed-off-by: Torbjörn Jansson <torbjorn.jansson@mbox200.swipnet.se>
(backported from af9a96d23eda3c9989e8b80c9ad2a9c41334aefd)