backlight: MacBookAir3,1(3,2) mbp-nvidia-bl support

Add support for the MacBookAir3,1 and MacBookAir3,2 to the mbp-nvidia-bl
driver.

Signed-off-by: Edgar (gimli) Hucek <gimli@dark-green.com>
Acked-by: Richard Purdie <rpurdie@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>