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>
(cherry picked from commit 73eeac49be566d389df728b5335f7146d03d2f90)
Link: https://github.com/openwrt/openwrt/pull/15919
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>