fujitsu-laptop: Add DMI callback for Lifebook S6420

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 <[email protected]>
Acked-by: Jonathan Woithe <[email protected]>
Signed-off-by: Len Brown <[email protected]>