ARM: dts: exynos: Remove unneeded address/size cells from fixed-clock on Exynos3250

xusbxti fixed-clock should not have address/size cells because it does
not have any children.  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>