Input: document i8042.debug in kernel-parameters.txt

i8042.debug parameter was missing in Documentation/kernel-parameters.txt.
Add it.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Documentation/kernel-parameters.txt

@@ -794,6 +794,7 @@ and is between 256 and 4096 characters. It is defined in the file
 			Defaults to the default architecture's huge page size
 			if not specified.
 
+	i8042.debug	[HW] Toggle i8042 debug mode
 	i8042.direct	[HW] Put keyboard port into non-translated mode
 	i8042.dumbkbd	[HW] Pretend that controller can only read data from
 			     keyboard and cannot control its state