ARM: dts: exynos: Move CPU OPP tables out of SoC node on Exynos5420

The cpus node is a top-level node, not inside the soc.  Therefore its
OPP tables should be there as well.

This also fixes the DTC warnings like:

    arch/arm/boot/dts/exynos5420.dtsi:46.37-109.5:
        Warning simple_bus_reg): /soc/opp_table0: missing or empty reg/ranges property

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>