update-dtc-source.sh generates the commit message after it runs
to completion. The word upstream is often used to refer to the
Linux kernel within barebox, so change the wording to refer
explicitly to dtc upstream.

Cc: Roland Hieber <r.hieber@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Acked-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>