gitmodules: add qemu.org vbootrom submodule

gitmodules: add qemu.org vbootrom submodule

The vbootrom module is needed for the new NPCM7xx ARM SoCs. The
vbootrom.git repo is now mirrored on qemu.org. QEMU mirrors third-party
code to ensure that users can always build QEMU even if the dependency
goes offline and so QEMU meets its responsibilities to provide full
source code under software licenses.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200915130834.706758-4-stefanha@redhat.com>