iio: pressure: Add driver for DLH pressure sensors

All Sensors DLH is series of low voltage digital pressure sensors.
Additionally to pressure value sensors deliver a temperature value.
Sensors can be accessed over I2C and SPI, this driver supports
only I2C access.

Signed-off-by: Tomislav Denis <tomislav.denis@avl.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>