evdev: add a rectangle to the touch arbitration

This enables us to specify the location that needs to be arbitrated, rather
than just disabling the whole device altogether. This patch just adds the
hooks, no implementation.

This is internal API only, one backend can specify an area in mm which gets
converted to device coordinates in the target device and arbitrated there.
Right now, everything simply passes NULL.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>