iio: chemical: Add support for sgp40 gas sensor

This patch set adds support for sensirion sgp40 gas sensor.

Many thanks for the in-depth reviews, especially of Jonathan who triggered some important improvements for v2:
- provide more precision for voc value; this in turn triggered the folling point:
- rework of e^x calculation with an optimization on the interesting range between e^(-6) and e^6
- restructure use of endian types
- use __packed structures for sent telegrams
- optimize usage of mutex
- optimize switch-cases
- replace attributes by read_raw() and write_raw() values
- add documentation in Documentation/ABI/testing/sysfs-bus-iio-sgp40 as well as in the source code

Andreas Klinger (2):
dt-bindings: iio: chemical: Add trivial DT binding for sgp40
iio: chemical: Add driver support for sgp40

 .../ABI/testing/sysfs-bus-iio-chemical-sgp40  |  31 ++
 .../devicetree/bindings/trivial-devices.yaml  |   2 +
 MAINTAINERS                                   |   6 +
 drivers/iio/chemical/Kconfig                  |  11 +
 drivers/iio/chemical/Makefile                 |   1 +
 drivers/iio/chemical/sgp40.c                  | 372 ++++++++++++++++++
 6 files changed, 423 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-chemical-sgp40
 create mode 100644 drivers/iio/chemical/sgp40.c

2.20.1