ARM: mvebu: Document the Device Tree binding for the Armada 388 SoC

Update the binding documentation of the Armada 38x Soc family with
the Armada 388.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>

Documentation/devicetree/bindings/arm/armada-38x.txt

@@ -15,6 +15,13 @@ Required root node property:
 
 compatible: must contain "marvell,armada385"
 
+In addition, boards using the Marvell Armada 388 SoC shall have the
+following property before the previous one:
+
+Required root node property:
+
+compatible: must contain "marvell,armada388"
+
 Example:
 
 compatible = "marvell,a385-rd", "marvell,armada385", "marvell,armada380";