sony-laptop: implement keyboard backlight support

Recent Vaios have the opportunity to control the keyboard backlight via
ACPI calls to the SNC device.
Introduce two module parameters to control how keyboard backlight should
be set at module loading (default to on and with 10 seconds timeout).

Tested-by: Marco Chiappero <marco@absence.it>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <mjg@redhat.com>