sony-laptop: support battery care functions

Allows limiting the maximum battery charge level to a selectable value
(100%, 80% and 50%).

[malattia@linux.it: group function specific variables in a struct, use
kstrtoul. Allow 0 to 100 values into sysfs files rounding to the actual
limit.]

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