`PICO_FLASH_SPI_CLKDIV` not mentioned in configuration defines

[recursivenomad]

The "Full List of SDK Configuration Defines" in the C/C++ SDK document does not list PICO_FLASH_SPI_CLKDIV, even though nearly every board header includes it.

https://datasheets.raspberrypi.com/pico/raspberry-pi-pico-c-sdk.pdf#config_parameters

[lurch]

I'm moving this to the pico-sdk repo, because that table is auto-generated from the PICO_CONFIG: comments in the SDK source code.

[lurch]

See also #2408

[kilograham]

fix merged into develop