ARM: dts: exynos: Add SCU device node to exynos4.dtsi

Exynos4 like other Cortex-A9 SoC's has a Snoop Control Unit(SCU)
and its SFR are used during SMP boot and S2R. Add SCU node to the device tree.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>