ARM: dts: Add syscon-{reboot, poweroff} nodes for exynos4

This patch adds syscon-{reboot, poweroff} nodes to allow the
generic syscon-{reboot, poweroff} driver to reset/poweroff exynos4 SoC.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>