MAINTAINERS: Move DT patchwork to kernel.org

The ozlabs.org PW instance is slow due to being geographically far away
from any of the maintainers and seems to have gotten slower as of late
(AI scrapers perhaps). The kernel.org PW also has some additional
features (i.e. pwbot) we want to use.

DT core patches also go into PW, so add the PW link for it.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>