This commit adds the Device Tree files for the Armada 39x family of
processors, as well as one Armada 398 Development Board.

Like for other Marvell EBU families, a common armada-39x.dtsi contains
the description of the common features of all Armada 39x SoCs, while
armada-390.dtsi and armada-398.dtsi respectively describe the
specificities of those SoCs.

Finally, an armada-398-db.dts file is added to describe the Armada 398
Development Board itself.

So far, the following features are supported:

 * SMP: dual Cortex-A9
 * Basic ARM IPs: SCU, timer, GIC, L2 cache
 * Basic Marvell IPs: pin-muxing, clocks, system controller, MBus
   controller, MPIC interrupt controller, timer, CPU reset for SMP,
   PMSU.
 * I2C
 * SPI
 * SDHCI
 * XOR
 * NAND
 * UART
 * PCIe

Additional features will be supported in the future.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>