arm64: mvebu: Add support for the Marvell Armada 3700 SoC

The Armada 3700 integrates the following interfaces (not complete list):
- Dual Cortex-A53 ARMv8
- USB 3.0
- SATA 3.0
- PCIe 2.0
- 2 x Gigabit Ethernet 1Gbps / 2.5Gbps
- ...

This patch adds basic support for this ARMv8 based SoC into U-Boot.
Future patches will integrate other device drivers and board support
for the Marvell DB-88F3720 development board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Wilson Ding <dingwei@marvell.com>
Cc: Victor Gu <xigu@marvell.com>
Cc: Hua Jing <jinghua@marvell.com>
Cc: Terry Zhou <bjzhou@marvell.com>
Cc: Hanna Hawa <hannah@marvell.com>
Cc: Haim Boot <hayim@marvell.com>