MAINTAINERS: Add Kumar to Device Tree Binding maintainers group

I'm tossing my hat into the ring of maintainers/reviewers for device tree
bindings based on history of dealing with DT on embedded PPC and starting
work on ARM SoCs.

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