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>

arch/riscv/boot/dts/canaan/k210.dtsi

@@ -259,7 +259,6 @@ spi2: spi@50240000 {
 					 <&sysclk K210_CLK_APB0>;
 				clock-names = "ssi_clk", "pclk";
 				resets = <&sysrst K210_RST_SPI2>;
-				spi-max-frequency = <25000000>;
 			};
 
 			i2s0: i2s@50250000 {