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>

drivers/iio/light/Kconfig

@@ -299,6 +299,16 @@ config MAX44000
 	 To compile this driver as a module, choose M here:
 	 the module will be called max44000.
 
+config MAX44009
+	tristate "MAX44009 Ambient Light Sensor"
+	depends on I2C
+	help
+	 Say Y here if you want to build support for Maxim Integrated's
+	 MAX44009 ambient light sensor device.
+
+	 To compile this driver as a module, choose M here:
+	 the module will be called max44009.
+
 config OPT3001
 	tristate "Texas Instruments OPT3001 Light Sensor"
 	depends on I2C

drivers/iio/light/Makefile

@@ -28,6 +28,7 @@ obj-$(CONFIG_SENSORS_LM3533)	+= lm3533-als.o
 obj-$(CONFIG_LTR501)		+= ltr501.o
 obj-$(CONFIG_LV0104CS)		+= lv0104cs.o
 obj-$(CONFIG_MAX44000)		+= max44000.o
+obj-$(CONFIG_MAX44009)		+= max44009.o
 obj-$(CONFIG_OPT3001)		+= opt3001.o
 obj-$(CONFIG_PA12203001)	+= pa12203001.o
 obj-$(CONFIG_RPR0521)		+= rpr0521.o