dt-bindings: marvell: document Turris Mox compatible

This adds the documentation for the Turris Mox compatible in armada-37xx
device-tree binding documentation.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>