backlight: adp5520: document sysfs attributes

Add documentation for sysfs interface of adp5520/adp5501 analog devices
backlight driver by reading code and looking through git commit logs.

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>