dt-bindings: mailbox: convert bcm2835-mbox bindings to YAML

Convert the DT binding document for bcm2835-mbox from .txt to YAML.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>