tests: templates/qemux86-64: add (empty) DTB

tests: templates/qemux86-64: add (empty) DTB

The JJB templates expect there to be a $DEVICE_DTB in various places, and
if not set here, will have a default value (from rpi3).

Change-Id: I3ae10a6fcf4f8f85dc49e45696a982ad85b29203
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10255
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>