SPI peripheral clock and frequency question

Hi Nordic Team,

I am using the nRF52840 for a project, and I am interfacing with an LCD using the SPIM3 peripheral.  I know there have been past posts on the Devzone which say that it is not recommended to use any other frequency for the SPI peripheral besides the ones provided (such as SPIM_FREQUENCY_FREQUENCY_M16), so I was wondering if there was any way to source the SPI peripheral from an external clock source instead? My LCD can use a 15MHz max clock, and I would prefer to run the LCD off 15MHz if possible for better performance, but the 16MHz frequency option is too fast and the 8MHz is too slow.  If not, is my only option to use either 8MHz or 16MHz, or would there be any other way to adjust the clock speed closer to 15MHz?

Thanks!

- Debbie