arm64: dts: exynos: Align OPP table name with dt-schema

Device tree nodes should have hyphens instead of underscores.  This is
also expected by the bindings.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200903191438.12781-6-krzk@kernel.org