dfflibmap: support flops with enable

[Ravenslofty]

Feature Description

This seems to be a notable oversight in this pass.

There are a few questions on hypotheticals though:

• should we include scan-chain flops which can be wired into enable flops? sky130 has these, but it also has flops with normal enables too.
• could this integrate with clockgate if a PDK does not provide an enable flop but provides a clock-gating primitive?

[povik]

should we include scan-chain flops which can be wired into enable flops?

I expect no, scan-chain flops shouldn't be misused to implement enables in ordinary logic. As we saw when we discussed this, scan-chain pins are annotated so we can avoid using them.