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.czSigned-off-by: NMarek Behún <marek.behun@nic.cz>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>