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

Marvell Armada 37xx Platforms Device Tree Bindings

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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";