platform/x86: ideapad-laptop: add keyboard backlight control support

platform/x86: ideapad-laptop: add keyboard backlight control support

On certain models it is possible to control/query the
keyboard backlight via the SALS/HALS ACPI methods. Add
support for that, and register an LED class device to expose
this functionality. Tested on: Lenovo YOGA 520-14IKB 80X8

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Link: https://lore.kernel.org/r/20210203215403.290792-27-pobrn@protonmail.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>