ARM: dts: exynos: Correct compatible of fixed clocks in Midas boards

The fixed clocks implemented by Samsung clock drivers use only one
compatible so drop the additional "fixed-clock" to fix dtbs_check
warnings like:

  arch/arm/boot/dts/exynos4412-i9300.dt.yaml: xxti: compatible:0: 'fixed-clock' was expected
    From schema: Documentation/devicetree/bindings/clock/fixed-clock.yaml
  arch/arm/boot/dts/exynos4412-i9300.dt.yaml: xxti: compatible: ['samsung,clock-xxti', 'fixed-clock'] is too long
  arch/arm/boot/dts/exynos4412-i9300.dt.yaml: xxti: compatible: Additional items are not allowed ('fixed-clock' was unexpected)
  arch/arm/boot/dts/exynos4412-i9300.dt.yaml: xxti: '#clock-cells' is a required property

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200830135200.24304-24-krzk@kernel.org