Gemmini Integration

[alonamid]

This PR integrates the Gemmini Matrix Multiplication acceleration unit to the Chipyard ecosystem.
This is a RoCC accelerator, and it is integrated very similarly to Hwacha.
It uses RoCC macros instead of dedicated custom assembled instructions.
This PR includes it in CI and in the example projects, and adds relevant documentation. It also bump esp-isa-sim to include the gemmini functional model.

[abejgonzalez]

Don't forget to update .circleci/check-commit.sh

[hngenc]

Lgtm

[jerryz123]

Can you squash away some of these "bump gemmini" messages. Otherwise fine

[abejgonzalez]

LGTM. On the sentence fixes, you can go ahead and add them then in the commit title you can put [ci skip] or [skip ci]. I'll let you merge this PR once done.