ARM: dts: exynos: Use pmu label in exynos4412

In order to use soc node, we need to reference soc child nodes by label
in exynos4412. Define pmu label in exynos4.dtsi. Use it in exynos4412
instead of redefining the node by full path.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>