dt-bindings: mailbox: Document armada-3700-rwtm-mailbox binding

This adds device tree binding documentation for the rWTM BIU mailbox
driver on the Armada 37xx SOC (EspressoBin, Turris Mox).

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>