Add support to STM HTS221 humidity + temperature sensor

http://www.st.com/resource/en/datasheet/hts221.pdf

- continuous mode support
- i2c support
- spi support
- trigger mode support
Signed-off-by: NLorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>