ARM: dts: exynos: Move Exynos5250 and Exynos5420 nodes under soc

The conventions is to put SoC devices under 'soc' node. In fact other
our DTSes (like exynos3250.dtsi or exynos5410.dtsi) already follow it.
Adjust exynos5250 and exynos5420 DTSI to follow this convention. This is
also necessary for the upcoming change in exynos5410.dtsi to inherit
from common exynos5.dtsi.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>