ipq40xx: dts: reduce the maximum SPI clock frequency to 24MHz

ipq40xx: dts: reduce the maximum SPI clock frequency to 24MHz

The IPQ4019 datasheet indicates that the maximum supported SPI
frequency is 25 MHz. My experiment on SKSpruce WIA3300-20 shows
that exceeding this threshold can lead to instability of SPI
peripheral. Limit the SPI clock frequency to the QSDK recommended
value 24MHz to enhance stability.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19744
Signed-off-by: Robert Marko <robimarko@gmail.com>