dt-bindings: bus: Document moxtet bus binding

This adds device tree binding documentation for the Moxtet bus, a bus
via which the different modules connected to the Turris Mox router can
be configured.

Link: https://lore.kernel.org/r/20190812161118.21476-3-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>