Add support for precision XTAL/TCXO

[mungewell]

For my own project, I needed a more accurate XTAL... obviously not everyone needs this, or wants to spend the extra money. However there is a nice 'hack'.

By adding 2 resistor footprints to connect to pin 2 of the XTAL, 'we' could configure the board to take either a passive XTAL or a (user performed modification) TCXO. Only extra one resistor, connecting to ground, would be fitted for mass production.

Would you consider adding this?

More info on what I am doing. https://github.com/mungewell/pico-timecode/issues/4