backlight: adp8860: document sysfs attributes

Add documentation for sysfs interface of adp8860 series backlight
devices by reading through code and git commits.
Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-class-backlight-adp8860

+sysfs interface for analog devices adp8860 backlight driver
+-----------------------------------------------------------
+
+The backlight brightness control operates at three different levels for the
+adp8860, adp8861 and adp8863 devices: daylight (level 1), office (level 2) and
+dark (level 3). By default the brightness operates at the daylight brightness
+level.
+
+What:		/sys/class/backlight/<backlight>/ambient_light_level
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	Michael Hennerich <michael.hennerich@analog.com>
+Description:
+		(RO) 13-bit conversion value for the first light sensor—high
+		byte (Bit 12 to Bit 8). The value is updated every 80 ms (when
+		the light sensor is enabled).
+
+
+What:		/sys/class/backlight/<backlight>/ambient_light_zone
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	Michael Hennerich <michael.hennerich@analog.com>
+Description:
+		(RW) Read or write the specific level at which the backlight
+		operates. Value "0" enables automatic ambient light sensing, and
+		values "1", "2" or "3" set the control to daylight, office or
+		dark respectively.
+
+
+What:		/sys/class/backlight/<backlight>/l1_daylight_max
+What:		/sys/class/backlight/<backlight>/l2_office_max
+What:		/sys/class/backlight/<backlight>/l3_dark_max
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	Michael Hennerich <michael.hennerich@analog.com>
+Description:
+		(RW) Maximum current setting for the backlight when brightness
+		is at one of the three levels (daylight, office or dark). This
+		is an input code between 0 and 127, which is transformed to a
+		value between 0 mA and 30 mA using linear or non-linear
+		algorithms.
+
+
+What:		/sys/class/backlight/<backlight>/l1_daylight_dim
+What:		/sys/class/backlight/<backlight>/l2_office_dim
+What:		/sys/class/backlight/<backlight>/l3_dark_dim
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	Michael Hennerich <michael.hennerich@analog.com>
+Description:
+		(RW) Dim current setting for the backlight when brightness is at
+		one of the three levels (daylight, office or dark). This is an
+		input code between 0 and 127, which is transformed to a value
+		between 0 mA and 30 mA using linear or non-linear algorithms.