iio:st_pressure:initial lps22hh sensor support

Initial support for ST LPS22HH pressure sensor. Datasheet:
http://www2.st.com/resource/en/datasheet/lps22hh.pdf



Features:
 * pressure, temperature data and timestamping channels
 * sampling frequency selection [1..200] Hz
 * interrupt based trigger
 * over I2C or SPI interface

Signed-off-by: mario tesi <mario.tesi@st.com>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>