ARM: mvebu: add kirkwood compatibles for cloudengine boards

This adds the compatible strings for Cloudengine PogoPlug E02 and
series 4. The former already has a devicetree in the kernel.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>