MAINTAINERS: remove swarren from DT bindings

I'm afraid I'm rather burned out w.r.t. DT bindings. As has probably
been evident, I don't have and haven't made the time to review many
recently. As such, remove myself from that MAINTAINERS entry.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>