ARM: dts: exynos: use hyphens in Exynos4 node names

Use hyphens instead of underscores in the Exynos4210 and Exynos4412 node
names which is expected by naming convention, multiple dtschema files
and pointed out by dtc W=2 builds.  Use also generic "ppmu" node name
for PPMU nodes to match Devicetree specification.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201105184506.215648-2-krzk@kernel.org