arm64: dts: exynos: Move fixed-clocks out of soc

arm64: dts: exynos: Move fixed-clocks out of soc

The XXTI fixed-clock is the input to the SoC therefore it should not be
inside the soc node.  This also fixes DTC W=1 warning:

    arch/arm64/boot/dts/exynos/exynos7.dtsi:90.17-94.5:
        Warning (simple_bus_reg): /soc/xxti: missing or empty reg/ranges property

While moving, change the name of the xxti node to match the generic type
of device (following DeviceTree specification).

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