devicetree: bindings: add DT binding for the Marvell Armada 3700 SoC family

The Marvell Armada 3700 is a family of ARMv8 CA53 SoCs. This commit
introduces the Device Tree binding that documents the top-level
compatible strings for Armada 3700 based platforms.
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: NRob Herring <robh@kernel.org>

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

+Marvell Armada 37xx Platforms Device Tree Bindings
+--------------------------------------------------
+
+Boards using a SoC of the Marvell Armada 37xx family must carry the
+following root node property:
+
+ - compatible: must contain "marvell,armada3710"
+
+In addition, boards using the Marvell Armada 3720 SoC shall have the
+following property before the previous one:
+
+ - compatible: must contain "marvell,armada3720"
+
+Example:
+
+compatible = "marvell,armada-3720-db", "marvell,armada3720", "marvell,armada3710";