package/dt: fix Kconfig package URL

The old package URL is no longer working. This commit updates it to
the one advertised at [1].

[1] https://github.com/RobinTMiller/dt/blob/b30eb7a123cfba900f26ceda413ea8ae49c3817e/html/dt.html#L33



Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>