Fine-tuning output pulse frequency in pulse generator mode
Problem: Trying to change the output frequency period (when configured to 10 MHz) by small value (e.g. 10 ps) doesn't change the actual output frequency.
Explanation: At 10 MHz, fine delay adjustment is not available due to design limitations (the HW design manual provides a detailed explanation).
Solution: Allow userspace to manually tune the local VCXO frequency when it's not locked to WR.