iio: Add SEMTECH SX9310/9311 sensor driver

Add SEMTECH SX9310/9311 driver.

The device has the following entry points:

Usual frequency:
- sampling_frequency
- sampling_frequency_available

Instant reading of current values for different sensors:
- in_proximity0_raw
- in_proximity1_raw
- in_proximity2_raw
- in_proximity3_comb_raw
and associated events in events/

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enrico Granata <egranata@chromium.org>
Signed-off-by: Daniel Campello <campello@chromium.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>