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>