riscv: dts: canaan: drop invalid spi-max-frequency

The spi-max-frequency is a property of SPI children, not the
controller:

  k210_generic.dtb: spi@50240000: Unevaluated properties are not allowed ('spi-max-frequency' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>