Support 17-bit-shifted PCIN input on DSP48E2

[gussmith23]

For Churchroad to implement wide multiplies, we need Lakeroad to support inputting to the PCIN port, which can be internally shifted by 17 bits.

[gussmith23]

2024-09-04

I am working on this in a PR, and I have a test for it. No luck yet. Some things I may want to do:

• Simplify the test so that it's literally just testing whether we can shift an input.
• Finally merge in the commits that clean up the input-specification, so that we can e.g. pass just a C input. Currently we can't do this, but I swear Vishal has commits somewhere that made this a lot cleaner.