ARM: dts: sunplus: sp7021: drop incorrect spi-max-frequency

spi-max-frequency is a property of a SPI slave device, not controller:

  sunplus-sp7021-demo-v3.dtb: spi@f480: Unevaluated properties are not allowed ('spi-max-frequency' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220810111131.281262-1-krzysztof.kozlowski@linaro.org