ARM: dts: exynos: Add soc node to exynos5440

The exynos5440 device tree is the only one left, which does not use
"soc" node. Add a "soc" node to exynos5440.dtsi in order to make it
consistent with other exynos DTS.
Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>