Are you talking about feeding the clock input on the PWM component? At the end of the PWM Datasheet is the AC specifications, if i remember correctly (i have not the datasheet at hand) the max. clock frequency at the clock input of the PWM component is around 64MHz, you can put more but the behavior is not stable.
If you are talking about the max. frequency the PWM component can generate is ~24MHz (limited by the max frequency the GPIOs can handle).
Hope it helps, check the PWM datasheet at the end is the table with some information not available on the first pages.