ARM: dts: exynos: drop unsupported desc-num in Exynos3250

There is no desc-num property for MMC nodes:

  exynos3250-monk.dtb: mmc@12510000: Unevaluated properties are not allowed ('desc-num' was unexpected)

Link: https://lore.kernel.org/r/20230120155404.323386-3-krzysztof.kozlowski@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>