devicetree: bindings: add DT binding for the Marvell Armada 39x SoC family

The Marvell Armada 39x is a family of two SoCs: the Armada 390 and the
Armada 398, with a slightly different number of interfaces. This
commit introduces the Device Tree binding that documents the top-level
compatible strings for Armada 39x based platforms.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

Documentation/devicetree/bindings/arm/armada-39x.txt

+Marvell Armada 39x Platforms Device Tree Bindings
+-------------------------------------------------
+
+Boards with a SoC of the Marvell Armada 39x family shall have the
+following property:
+
+Required root node property:
+
+ - compatible: must contain "marvell,armada390"
+
+In addition, boards using the Marvell Armada 398 SoC shall have the
+following property before the previous one:
+
+Required root node property:
+
+compatible: must contain "marvell,armada398"
+
+Example:
+
+compatible = "marvell,a398-db", "marvell,armada398", "marvell,armada390";