ARM: mvebu: add Device Tree for the Armada 375 DB board

The Armada 375 DB board is the development board from Marvell for the
Armada 375 SoC. This commit adds a Device Tree description for this
board, which enables the following features:

 * I2C buses
 * SDIO
 * Serial port
 * SPI bus, with a SPI flash. Note that the SPI bus is disabled by
   default, because it conflicts with the NAND, and can only work if
   the board boots out of SPI. Since most boards are shipped to boot
   out of NAND, we're default to having the SPI bus disabled.
 * PCIe interfaces

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>