arm: Add arch/arm/dts/Makefile specifically to MAINTAINERS

In order to reduce the number of people that are cc'd on a patch for
simply touching arch/arm/dts/Makefile (which is a big common file) add
an entry specifically to MAINTAINERS under the ARM entry.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>