drivers: spi: use CPU clock frequency in configuration on CC13XX/CC26XX
drivers: spi: use CPU clock frequency in configuration on CC13XX/CC26XX

The SPI peripheral should be configured using the CPU clock speed and
not the system clock speed. This used to be fine because they were the
same before #19232, but now that the system clock is RTC-based (which
has a different frequency), we can no longer make that assumption.

Signed-off-by: Vincent Wan <0da4a49376daf88459b96a63301ecd5aff2f3b80@linaro.org>

Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr