panasonic-laptop: add Panasonic Let's Note laptop extras driver v0.94

This is a driver for ACPI extras such as hotkeys and backlight
brightness control on various Panasonic "Let's Note" series laptop
computers.

It exports the backlight via the backlight class device API,
and the hotkeys as input event device.  Some more esoteric
items like number of installed batteries are exported via sysfs
device attributes.

Hotkey events also generate old-style ACPI enents through
/proc/acpi/event to interoperate with current versions of acpid.

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>