I did play with the SPI Speed a while ago. At the time the SPI speed seemed to be somewhat broken. Here are my results, note that this is an older kernel, Debian, not Yocto, and the bug (feature?) may have been fixed, I haven't retried it lately.
Requested Speed measured frequency
10,000 1us 1MHz
100,000 1us 1MHz
1,000,000 200ns 5MHz
2,000,000 200ns 5MHz
10,000,000 62.5ns 16MHz
50,000,000 20ns 50MHz
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