arm64: dts: Add ARM PMU node for exynos7

This patch adds ARM Performance Monitor Unit dt node for exynos7.
PMU provides various statistics on the operation of the CPU and
memory system at runtime, which are very useful when debugging or
profiling code. This enables the same.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
[krzk: Squashed with "Add level for cpu dt node for exynos7"]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>