arm: mvebu: support for the PlatHome OpenBlocks AX3-4 board

This platform, available in Japan from PlatHome, has a dual-core
Armada XP, the MV78260. For now, only the two serial ports and the
three front LEDs are supported. Support for SMP, network, SATA, USB
and other peripherals will be added as drivers for them become
available for Armada XP in mainline.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

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This is 3.8 material.

Changes since v2:

 * Renamed the .dts file to armada-xp-openblocks-ax3-4.dts

 * Removed the compatible string from armada-370-xp.c (which now only
   lists the common SoC compatible string)

Changes since v1:

 * Renamed the board to OpenBlocks AX3-4, since there is a variant
   called AX3-2 which has less RAM, and no mini PCIe port. Requested
   by Andrew Lunn.

 * Fix the amount of memory to 3 GB. In fact, the board has 1 GB
   soldered, and 2 GB in a SODIMM slot (which is therefore
   removable). But as the board is delivered as is, we'll assume it
   has 3 GB of memory by default.