MAINTAINERS: remove myself from DT bindings entry

For quite a while Rob has been handling DT binding maintenance, and I
haven't had the time to review bindings outside of a few targetted
cases. Given that, I think being listed in MAINTAINERS is more
misleading than helpful.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>