ARM: mvebu: add Device Tree files for Armada 39x SoC and board

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>