ARM: dts: exynos: use hyphens in MFC reserved memory node names

Use hyphens instead of underscores in the MFC reserved memory 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/20201105184506.215648-5-krzk@kernel.org