docs: lp855x-driver.rst: add it to the driver-api book

The content of this file is intended for backlight Kernel
developers.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/backlight/lp855x-driver.rst → Documentation/driver-api/backlight/lp855x-driver.rst

-:orphan:
-
 ====================
 Kernel driver lp855x
 ====================

Documentation/driver-api/index.rst

@@ -66,6 +66,7 @@ available subsections can be seen below.
    soundwire/index
    fpga/index
    acpi/index
+   backlight/lp855x-driver.rst
    generic-counter
 
 .. only::  subproject and html

MAINTAINERS

@@ -15964,7 +15964,7 @@ F:	sound/soc/codecs/isabelle*
 TI LP855x BACKLIGHT DRIVER
 M:	Milo Kim <milo.kim@ti.com>
 S:	Maintained
-F:	Documentation/backlight/lp855x-driver.rst
+F:	Documentation/driver-api/backlight/lp855x-driver.rst
 F:	drivers/video/backlight/lp855x_bl.c
 F:	include/linux/platform_data/lp855x.h