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: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>