iio: humidity: Add TI HDC20x0 support

iio: humidity: Add TI HDC20x0 support

Add driver support for HDC2010/2080 series devices and sysfs
documentation for their heater element.

HDC2010 is an integrated high-accuracy humidity and temperature sensor
with very low power consumption. The device includes a resistive heating
element. The temperature range is -40C to 125C with 0.2C
accuracy. Humidity measurement is 0 to 100% with 2% RH accuracy.

Signed-off-by: Eugene Zaikonnikov <ez@norphonic.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>