docs: lp855x-driver.txt: convert to ReST and move to kernel-api

This small file seems to be an attempt to start documenting
backlight drivers.

It contains descriptions of the controls for the driver
with could sound as an somewhat user-faced description, but
it's main focus is to describe, instead, the data that should
be passed via platform data and some driver-specific stuff.

While this is not part of the driver-api book, mark it as
:orphan:, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>