drivers: i2c: use CPU clock frequency in configuration on CC13XX/CC26XX

The I2C peripheral should be configured using the CPU clock frequency
and not the system clock frequency. 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.

Fixes #20480

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>