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 <[e-mail]>
Signed-off-by: Conor Dooley <[e-mail]>

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

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