iio: acpi: Add support for ACPI0008 Ambient Light Sensor

This driver adds the initial support for the ACPI Ambient Light Sensor
as defined in Section 9.2 of the ACPI specification (Revision 5.0) [1].

Sensors complying with the standard are exposed as ACPI devices with
ACPI0008 as hardware ID and provide standard methods by which the OS
can query properties of the ambient light environment the system is
currently operating in.

This driver currently allows only to get the current ambient light
illuminance reading through the _ALI method, but is ready to be
extended extended to handle _ALC, _ALT and _ALP as well.

[1] http://www.acpi.info/DOWNLOADS/ACPIspec50.pdf

Signed-off-by: Martin Liska <marxin.liska@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/light/Kconfig b/drivers/iio/light/Kconfig
index 0ce46fd..e6198b7 100644
--- a/drivers/iio/light/Kconfig
+++ b/drivers/iio/light/Kconfig
@@ -5,6 +5,19 @@
 
 menu "Light sensors"
 
+config ACPI_ALS
+	tristate "ACPI Ambient Light Sensor"
+	depends on ACPI
+	select IIO_BUFFER
+	select IIO_TRIGGERED_BUFFER
+	select IIO_KFIFO_BUF
+	help
+	 Say Y here if you want to build a driver for the ACPI0008
+	 Ambient Light Sensor.
+
+	 To compile this driver as a module, choose M here: the module will
+	 be called acpi-als.
+
 config ADJD_S311
 	tristate "ADJD-S311-CR999 digital color sensor"
 	select IIO_BUFFER