Input: define INPUT_PROP_ACCELEROMETER behavior

Spell out what this property means to userspace. If the property is set, all
directional axes must be accelerometer axes, any other axes are left as-is.
This allows an accelerometer device to e.g. have an ABS_WHEEL.

It is not permitted to mix normal directional axes and accelerometer axes on
the same device node.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/input/event-codes.txt

@@ -294,6 +294,12 @@ accordingly. This property does not affect kernel behavior.
 The kernel does not provide button emulation for such devices but treats
 them as any other INPUT_PROP_BUTTONPAD device.
 
+INPUT_PROP_ACCELEROMETER
+-------------------------
+Directional axes on this device (absolute and/or relative x, y, z) represent
+accelerometer data. All other axes retain their meaning. A device must not mix
+regular directional axes and accelerometer axes on the same event node.
+
 Guidelines:
 ==========
 The guidelines below ensure proper single-touch and multi-finger functionality.