MAINTAINERS: Add DT bindings directory to mailbox

DT bindings related to mailbox drivers are typically picked by the
mailbox maintainer, so add the binding folder to the maintainers entry
to make sure get_maintainer finds it.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>