ARM: dts: exynos: Correct compatible for Exynos5260 GIC

Exynos5260 SoCs have ARM GIC 400.  Correct the compatibles to match
dtschema and fix the dtbs_check warnings like:

  arch/arm/boot/dts/exynos5260-xyref5260.dt.yaml: interrupt-controller@10481000:
    compatible: ['arm,cortex-a15-gic', 'arm,cortex-a9-gic'] is not valid under any of the given schemas

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200901075417.22481-3-krzk@kernel.org