target/armv7m_trace: Improve SWO frequency auto-detection
The SWO frequency auto-detection with J-Link adapters does not work
properly in the current implementation. This is because the trace layer
has only information about the highest possible SWO frequency supported
by the adapter. With that the trace layer calculates the SWO prescaler
which usually leads to a frequency deviation greater than what is
permitted by J-Link adapters.
Move the calculation of the SWO prescaler from the trace layer into the
trace configuration of the adapter to overcome this problem.
The adapter has the necessary information to choose a suitable SWO
frequency and calculate the corresponding prescaler that complies with
the maximum allowed frequency deviation.