Clean up PA0 for timing input from Pounder

[jordens]

sinara-hw/Pounder#76 (comment)

For Stabilizer v1.2 this just means leave J10:18 to PA0 (as now) and clean up the PA0 wire for crosstalk and impedance and add DNP parallel AC termination. The rest can be done on Pounder, I think.

[gkasprow]

added AC termination

fixed trace impedance