iio: pressure: add support to LPS33HW and LPS35HW

add support to STMicroelectronics LPS33HW and LPS35HW pressure sensors
to st_pressure framework

http://www.st.com/resource/en/datasheet/lps33hw.pdf
http://www.st.com/resource/en/datasheet/lps35hw.pdf


Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>