MAINTAINERS: Refactor device tree maintainership

Device tree bindings require a lot more attention than they used to.
We've got a group of volunteers willing to take over maintaining
bindings. This patch adds them to the MAINTAINERS file.

This group still needs to work out a process for maintainership and how
they are going to work together. I recommend that they set up a shared
tree on git.kernel.org that they each have commit access to similar to
the tip tree or the arm-soc tree, but it is up to them.

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