iio: Add AS3935 lightning sensor support

AS3935 chipset can detect lightning strikes and reports those back as
events and the estimated distance to the storm.
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-proximity-as3935

+What		/sys/bus/iio/devices/iio:deviceX/in_proximity_raw
+Date:		March 2014
+KernelVersion:	3.15
+Contact:	Matt Ranostay <mranostay@gmail.com>
+Description:
+		Get the current distance in meters of storm (1km steps)
+		1000-40000 = distance in meters
+
+What		/sys/bus/iio/devices/iio:deviceX/sensor_sensitivity
+Date:		March 2014
+KernelVersion:	3.15
+Contact:	Matt Ranostay <mranostay@gmail.com>
+Description:
+		Show or set the gain boost of the amp, from 0-31 range.
+		18 = indoors (default)
+		14 = outdoors

drivers/iio/Kconfig

@@ -74,6 +74,7 @@ if IIO_TRIGGER
    source "drivers/iio/trigger/Kconfig"
 endif #IIO_TRIGGER
 source "drivers/iio/pressure/Kconfig"
+source "drivers/iio/proximity/Kconfig"
 source "drivers/iio/temperature/Kconfig"
 
 endif # IIO

drivers/iio/Makefile

@@ -24,5 +24,6 @@ obj-y += light/
 obj-y += magnetometer/
 obj-y += orientation/
 obj-y += pressure/
+obj-y += proximity/
 obj-y += temperature/
 obj-y += trigger/

drivers/iio/proximity/Kconfig

+#
+# Proximity sensors
+#
+
+menu "Lightning sensors"
+
+config AS3935
+	tristate "AS3935 Franklin lightning sensor"
+	select IIO_BUFFER
+	select IIO_TRIGGERED_BUFFER
+	depends on SPI
+	help
+	  Say Y here to build SPI interface support for the Austrian
+	  Microsystems AS3935 lightning detection sensor.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called as3935
+
+endmenu

drivers/iio/proximity/Makefile

+#
+# Makefile for IIO proximity sensors
+#
+
+# When adding new entries keep the list in alphabetical order
+obj-$(CONFIG_AS3935)		+= as3935.o