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>