ARM: dts: exynos: use hyphens in Exynos5 node names

Use hyphens instead of underscores in the Exynos5250 and Exynos542x node
names which is expected by naming convention, multiple dtschema files
and pointed out by dtc W=2 builds.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201105184506.215648-4-krzk@kernel.org