arm64: dts: qcom: ipq6018: drop incorrect SPI bus spi-max-frequency

The spi-max-frequency property belongs to SPI devices, not SPI
controller:

  ipq6018-cp01-c1.dtb: spi@78b5000: Unevaluated properties are not allowed ('spi-max-frequency' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230416123730.300863-1-krzysztof.kozlowski@linaro.org