arm: mvebu: support for the new Armada XP development board(DB-MV784MP-GP)

This is the new Armada XP evaluation board from Marvell. It comes with
a RS232 port over USB, a SATA link, an internal SSD, 4 Ethernet
Gigabit links.

Support for USB (Host and device), SDIO, PCIe will be added as drivers
when they become available for Armada XP in mainline.

Tested-by: Simon Guinot <simon.guinot@sequanux.org>
Tested-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>