dt-bindings: mmc: mmc-spi-slot: drop unneeded spi-max-frequency

spi-max-frequency comes from spi-peripheral-props.yaml.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220727164202.385531-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>