dt-bindings: firmware: Document cznic,turris-mox-rwtm binding

This adds device tree binding documentation for the driver communicating
with the rWTM firmware on Turris Mox.

Link: https://lore.kernel.org/r/20190822014318.19478-2-marek.behun@nic.cz


Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>