This reverts commit 21c67bc8.
DCM is much more jittery than PLL_BASE:
DCM_SP:
125MHz -> 62.5MHz: pk-to-pk jitter: 300ps
125MHz -> 125MHz: pk-to-pk jitter: 200ps
20MHz -> 62.5MHz: pk-to-pk jitter: 1772ps (!!!!)

PLL_BASE:
125MHz -> 62.5MHz: pk-to-pk jitter: 185ps
125MHz-> 125MHz: pk-to-pk jitter 161ps
20MHz -> 62.5MHz: pk-to-pk jitter 417ps