Support for more DSP modes

[gregdavill]

Currently trellis only offers MULT18X18D primitives for the DSP blocks.

Internally these DSP blocks appear quite configurable, and offer a 9x9bit mode.
Official tooling enables this at the verilog level with a MULT9X9D primitive.

I'm not sure on exactly the current state of documentation of the DSP bits, I assume enabling this will require changes at the fuzzing level all the way up the toolchain flow.

[qcabanes]

I am also interested in the support of MULT9X9D mode, because it means I could use twice as much DSP than with MULT18X18D. Is there anyone currently working on it? Otherwise, I would be glad to try to contribute for this part.