ARM: mvebu: add Device Tree for the Armada 385 RD board

The Armada 385 RD board is the reference design board from Marvell
for the Armada 385 SoC. This commit adds a Device Tree description for
this board, which enables the following features:

 * Network interfaces
 * I2C bus
 * Serial port
 * SPI bus, with a SPI flash
 * PCIe interface

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>