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>

Documentation/devicetree/bindings/arm/marvell/armada-37xx.txt

@@ -48,3 +48,11 @@ avs: avs@11500 {
 	compatible = "marvell,armada-3700-avs", "syscon";
 	reg = <0x11500 0x40>;
 }
+
+
+CZ.NIC's Turris Mox SOHO router Device Tree Bindings
+----------------------------------------------------
+
+Required root node property:
+
+ - compatible: must contain "cznic,turris-mox"