misc: add dell-laptop driver

Add a driver for controlling Dell-specific backlight and rfkill interfaces.
This driver makes use of the dcdbas interface to the Dell firmware to
allow the backlight and rfkill interfaces on Dell systems to be driven
through the standardised sysfs interfaces.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Matt Domsch <Matt_Domsch@dell.com>
Cc: Ivo van Doorn <ivdoorn@gmail.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>