Add dependabot to auto-update submodules

[abejgonzalez]

Opening this to spawn discussion on how/should we use this to update submodules. I think this is nice, but we should 1. configure how often it checks for things and 2. what it should update v.s. what we should do ourselves. In any case, the PRs that it opens still have to be manually approved (see FireSims PRs from dependabot: https://github.com/firesim/firesim/pulls)

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[jerryz123]

Will CI run on these PRs? A big limitation with the CIRCT action is that it doesn't run our CI.

[abejgonzalez]

Yup. It will run the CI by default.

[jerryz123]

I'm willing to try this out.

[abejgonzalez]

Small update to instead ignore selected submodules instead of listing them all out. Merging since this doesn't affect CI.