hw/misc: Add a config switch for the "unimplemented" device

The device is only used by some few boards. Let's use a proper Kconfig
switch so that we only compile this code if we really need it.

Message-Id: <20190817101931.28386-8-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>