iio:st_pressure:initial lps22hb sensor support

Initial support for ST LPS22HB pressure sensor. Datasheet:
http://www2.st.com/resource/en/datasheet/lps22hb.pdf



Features:
* pressure data and timestamping channels
* sampling frequency selection
* interrupt based trigger
* over I2C or SPI

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>