soc: samsung: pmu: Populate children syscon nodes

The syscon poweroff and restart nodes logically belong to the Power
Management Unit so populate possible children.

This also requires providing compatibles for Exynos5410 and Exynos7 so
the PMU device and its children will be instantiated for them as well.
Just like Exynos5433, these chipsets are not yet supported by the PMU
driver.

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