backlight: lm3639: document sysfs attributes

Add documentation for sysfs interfaces of Texas Instruments lm3639
backlight + flash led driver chip by looking through git commits and
reading code.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-class-backlight-lm3639

+sysfs interface for Texas Instruments lm3639 backlight + flash led driver chip
+------------------------------------------------------------------------------
+
+What:		/sys/class/backlight/<backlight>/bled_mode
+Date:		Oct, 2012
+KernelVersion:	v3.10
+Contact:	dri-devel@lists.freedesktop.org
+Description:
+		(WO) Write to the backlight mapping mode. The backlight current
+		can be mapped for either exponential (value "0") or linear
+		mapping modes (default).