Ignore PLLs that are unavailable in the current package

[cliffordwolf]

Some of the iCE40 devices have a PLL disabled in certain chip packages. Current git head of Project IceStorm adds this information to the chipdb file.

1K ChipDB file:

.extra_cell 6 0 PLL
LOCKED cb121 cb81 cm49 swg16tr cm36 qn48 vq100

8K ChipDB file:

.extra_cell 16 33 PLL
LOCKED cm81:4k cm81

In commit 45ea318 I added a quick hack to simply ignore those entries.

Ultimately the PLLs should be made unavailable to the placer if arachne-pnr is called with a chip package that has the PLL marked as locked. But I'm not sure about the best way to add this to arachne-pnr.

[daveshah1]

Should (finally) be fixed in #91