MAINTAINERS: Drop Frank Rowand from DT maintainership

Frank retired a while back and surprisingly has found better things to
do than DT maintainership. Thank you Frank for all of your help with DT
for many years.

Link: https://lore.kernel.org/r/20240122211501.1719374-1-robh@kernel.org


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