ARM: dts: exynos: Remove snps, dwc2 compatible in Exynos3250

The binding for DesignWare HS OTG USB 2.0 in Samsung SoCs take only one
compatible.  This fixes dtbs_check warnings like:

  arch/arm/boot/dts/exynos3250-artik5-eval.dt.yaml: hsotg@12480000:
    compatible: ['samsung,s3c6400-hsotg', 'snps,dwc2'] is not valid under any of the given schemas (Possible causes of the failure):

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200829172532.29358-4-krzk@kernel.org