MAINTAINERS: Add Frank Rowand as DT maintainer

Frank has agreed to step up and help with DT core code maintainership.
At the same time, Grant is taking a step back from active maintainership
responsibilities. Add Frank to the device tree core code entry and
shuffle Grant to the end of the list. In a few releases time Grant will
be removed entirely.

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