sony-laptop: add touchpad enable/disable function

This setting is stored in the EC and available across reboots.

[malattia@linux.it: group function specific variables in a struct, use
kstrtoul]

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