fujitsu-laptop: Add DMI callback for Lifebook S6420

The Lifebook S6420 is the ICH9M-based follow-up to the S6410. The application panel
contains the following keys: lock, mobility center, eco, info.
Whilst key 4 might be more appropriate for help then key 2, I've done things the
S6410 way. I can confirm that backlight control is functional, and that the lock key
activates the Gnome screensaver as expected.

Signed-off-by: Tony Vroon <tony@linx.net>
Acked-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Signed-off-by: Len Brown <len.brown@intel.com>