sony-laptop: cache handles and report them via sysfs

Avoid calling into acpi each time we need to lookup a method handle
and report the available handles to ease collection of information when
debugging issues. Also move initialization of the platform driver
earlier to allow adding files from other setup functions.

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