The SWD frequency depends a lot of what kind of programming lines you have, and will be lower if there is a lot of capacitance, difference in lengths etc.
The SWD interface speed self regulates, for an indication on what is possible refer to the programming speed of for example SEGGER programmers.
We only specify that the clock speed has to be atleast 125 kHz. See the section in the reference manual on the debugger interface for more information.
