arm64: dts: exynos: Move GPU under /soc node for Exynos7

Mali GPU hardware module is a standard hardware module integrated to
Exynos7 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>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>