nextpnr-ice40 fails with:
```
Info: Placing PLLs..
ERROR: PLL 'reset_gen_inst.pll_inst' couldn't be placed anywhere, no
suitable BEL found.
PLL bel 'X12/Y31/pll_3' cannot be used as it conflicts with input
'interface_ch552_cts$sb_io' on pin 'B3'.
```