arm64: dts: add the Marvell Armada 3700 family and a development board

Add initial dtsi files to support Marvell Armada 3700 SoC with Cortex-A53
CPUs. There are two members in this family: the Armada 3710 (Single CPU)
and the Armada 3720 (Dual CPUs).

It also adds a dts file for the Marvell Armada 3720 DB board.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>