riscv: dts: canaan: fix kd233 display spi frequency

The binding for the ili9341 specifies a const spi-max-frequency of 10
MHz but the kd233 devicetree entry has it listed at 15 Mhz.
Align the devicetree with the value in the binding.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220705215213.1802496-9-mail@conchuod.ie


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>