ARM: dts: exynos: adjust node names to DT spec in Exynos3250 boards

The Devicetree specification expects device node names to have a generic
name, representing the class of a device.  Also the convention for node
names is to use hyphens, not underscores.

No functional changes.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201027170947.132725-2-krzk@kernel.org