efi_loader: support for Ctrl() device path node

* Add the definitions for Ctrl() device path nodes.
* Implement Ctrl() nodes in the device path to text protocol.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>