MAINTAINERS: Add dt-bindings to mailbox entry

The MAINTAINER entry for the MAILBOX framework does not cover the
dt-bindings and as such Jassi is not among the recipients for such
patches.

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