arm64: dts: exynos: Move syscon poweroff and restart nodes under the PMU

The PMU node is the actual block responsible for power management,
including typical Exynos on/off/restart procedures.  Therefore the
syscon poweroff and restart nodes logically belong to it.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>