ideapad: add hotkey support

Hotkey enabled by this patch:
  Fn+F3: Video mode switch
  Fn+F5: software rfkill for wifi

For some ideapad when push Fn+F3, hardware generates Super-P keys, those key
will not be enabled by this patch.

Thanks for Dave Hansen report the problem. If CONFIG_INPUT_SPARSEKMAP is not
set, when building, you will have error message:

  ERROR: "sparse_keymap_setup" [drivers/platform/x86/ideapad-laptop.ko] undefined!
  ERROR: "sparse_keymap_free" [drivers/platform/x86/ideapad-laptop.ko] undefined!
  ERROR: "sparse_keymap_report_event" [drivers/platform/x86/ideapad-laptop.ko] undefined!

To select INPUT_SPARSEKMAP solve this issue.

Ref: http://lkml.org/lkml/2010/12/2/340



Signed-off-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>