ARM: dts: mvebu: Add device tree for db-dxbc2 and db-xc3-24g4xg boards

These boards are Marvell's evaluation boards for the 98DX4251 and
98DX3336 SoCs.

[gregory.clement@free-electrons.com: fix topic and update Makefile]
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>