Raspberry Pi Pico change SPI flash clock divider

Is it currently possible to change the SPI flash clock divider for the Raspberry Pi Pico? The way to do this with the Pico SDK would be to define `PICO_FLASH_SPI_CLKDIV`, but this doesn't seem to have any effect here.

Start by locating the Raspberry Pi Pico flash and SPI initialization, then compare how this project handles the Pico SDK setting PICO_FLASH_SPI_CLKDIV. Confirm whether changing the setting affects the SPI flash clock divider; done means the requested configuration works or the issue is documented as unsupported, with a verification path.

Written by the indexing model from the issue text.