ARM: dts: exynos: Move G2D node to exynos5.dtsi

Current the node is only defined in the exynos5250 DT, while
the corresponding SYSMMUs are also to be found in the exynos5420
DT. Move the node to exynos5 and only setup the SYSMMUs in the
corresponding DT.

Disable the node by default, since exynos5410 also includes
the dtsi, but currently does not define the G2D clock.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
[mszyprow: rephrased commit message]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>