ARM: dts: Add PMU to exynos5410

Exynos initialization code now relies on obtaining the PMU address,
so prepare a PMU node for Exynos5410.

Fixes: fce9e5bb ("ARM: EXYNOS: Add support for mapping PMU base
address via DT")

Signed-off-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>