ARM: dts: exynos: Move fixed-clocks out of soc on Exynos3250

The three fixed-clocks (xusbxti, xxti and xtcxo) are inputs to the
Exynos3250 therefore they should not be inside the soc node.  This also
fixes DTC W=1 warning:

    arch/arm/boot/dts/exynos3250.dtsi:112.21-139.5:
        Warning (simple_bus_reg): /soc/fixed-rate-clocks: missing or empty reg/ranges property

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