MAINTAINERS: Update DT website and git repository

Per Grant, secretlab.ca is defunct and he has moved his tree to
kernel.org so update the DT website and git tree. devicetree.org needs
work, but is better than "Internal Server Error" that secretlab.ca
returns.

Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>