asus-laptop: switch to sparse keymap library

This patch is based on Dmitry Torokhov's patch with
some modifications and cleanups.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
diff --git a/drivers/platform/x86/Kconfig b/drivers/platform/x86/Kconfig
index f526e73..7f5a222 100644
--- a/drivers/platform/x86/Kconfig
+++ b/drivers/platform/x86/Kconfig
@@ -59,6 +59,7 @@
 	select NEW_LEDS
 	select BACKLIGHT_CLASS_DEVICE
 	depends on INPUT
+	select INPUT_SPARSEKMAP
 	---help---
 	  This is the new Linux driver for Asus laptops. It may also support some
 	  MEDION, JVC or VICTOR laptops. It makes all the extra buttons generate