ramips: limit max spi clock frequency to 50 MHz

ramips: limit max spi clock frequency to 50 MHz

In the past few years, we have received several reports about SPI
Flash not working properly. This is caused by excessively fast
clock frequency. It's really annoying to fix them one by one. Let's
reduce these aggressive frequencies to 50 MHz. This is a safe and
suggested value in the vendor SDK.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>