platform: x86: dell-laptop: Add support for keyboard backlight

This patch adds the support for the configuration of the keyboard
backlight on supported Dell laptops.

With this patch it is possible to set:
* keyboard backlight level
* timeout after which the backlight will be automatically turned off
* input activity triggers (keyboard, touchpad, mouse) that enable the backlight
* ambient light settings

The settings are exposed via /sys/class/leds/dell::kbd_backlight/

The code is based on the newly released documentation by Dell in the
libsmbios project.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>