Allow use of fixed ef_sc_ cells for li1 density rules.

[donnie-j]

The scripts have some hard coded fill and decap cells, in platforms/sky130A/fill_cells.yml and sky130_fd_sc_hd.v. These have to be in sync or the netlist checks fail.

Suggestion, instead of hacking these: Can it be pulled out of the PDK?

Perhaps @antonblanchard has a quick fix.