arm: mvebu: support for the Globalscale Mirabox board

This platform, available from Globalscale has an Armada 370. For now,
only the serial port is supported. Support for network, USB and other
peripherals will be added as drivers for them become available for
Armada 370 in mainline.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
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This is 3.8 material.

Changes from original version posted by Gregory:
 * Renamed .dts file to armada-370-mirabox.dts
 * Change compatible string to 'globalscale,mirabox'
 * Remove compatible string from armada-370-xp.c
 * Removed references to MBX0001