ARM: dts: exynos: Move syscon reboot/poweroff to common dtsi

All Exynos SoCs have the same syscon reboot and poweroff device nodes so
there is no need to duplicate the same on each SoC dtsi and can be moved
to a common dtsi that can be included by all the SoCs dtsi files.

Suggested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung,com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>