ARM: dts: exynos: Move Mali400 GPU node to "/soc"

Mali400 GPU hardware module is a standard hardware module integrated to
Exynos3210/4210/4412 SoCs, so it should reside under the "/soc" node.
The only SoC components which are placed in the DT root, are those, which
are a part of CPUs: like ARM architected timers and ARM performance
measurement units.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>