dt-bindings: marvell: a38x: add kobol helios-4 board

Add DT compatible for the helios-4 nas by Kobol, which is already used
in-tree.

This product shares a common system on module with the solidrun armada
388 clearfog boards, however it is not easily described in a single
list due to their extra "solidrun,clearfog-a1" compatible string.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>