efi: Add a separate maintainer entry for the app

Separate this out slightly from the payload, with a new entry.

We might consider renaming EFI PAYLOAD to EFI LOADER, but that would
require quite a lot of file changes.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>