MAINTAINERS: change my DT related maintainer address

Filtering capabilities on my work email are pretty much non-existent and this
has turned out to be something of a firehose...

Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>