ARM: dts: exynos: use hyphens in Exynos3250 node names

Use hyphens instead of underscores in the Exynos3250 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/20201105184506.215648-1-krzk@kernel.org