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>

drivers/iio/light/Makefile

@@ -3,6 +3,7 @@
 #
 
 # When adding new entries keep the list in alphabetical order
+obj-$(CONFIG_ACPI_ALS)		+= acpi-als.o
 obj-$(CONFIG_ADJD_S311)		+= adjd_s311.o
 obj-$(CONFIG_AL3320A)		+= al3320a.o
 obj-$(CONFIG_APDS9300)		+= apds9300.o