iio: light: add driver for MAX44009

The MAX44009 is a low-power ambient light sensor from Maxim
Integrated. It differs from the MAX44000 in that it doesn't have
proximity sensing and that it requires far less current (1 micro-amp
vs 5 micro-amps). The register mapping and feature set between the
two are different enough to require a new driver for the MAX44009.

Developed and tested with a BeagleBone Black and UDOO Neo (i.MX6SX)

Supported features:

* Reading lux (processed value)

* Rising and falling illuminance threshold
  events

* Configuring integration time

https://datasheets.maximintegrated.com/en/ds/MAX44009.pdf



Signed-off-by: Robert Eshleman <bobbyeshleman@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>