Add missing CSRs

[cmd05]

Adds basic information for missing CSRs necessary for #1209. List of CSRs: sie, dscratch0, dscratch1

Based on:

• https://people.eecs.berkeley.edu/~krste/papers/riscv-privileged-v1.9.1.pdf
• riscv/riscv-isa-manual@87a1e5f

[ThinkOpenly]

It appears that sedeleg and sideleg were also defined by the now-defunct "N" extension, so please remove those, too.

That leaves sie (defined by "S" extension), dscratch0 and dscratch1 (defined by "Sdext" extension).

Looks like to get these new CSRs to pass the CI tests, we'll need to add the fields.

• For sie, that points to scause for interrupt values. scause is defined in UDB and has some code that seems to beg for expansion, but I don't see anything that performs that expansion. If you want to replicate the incomplete implementation in scause into sie, that's fine for now, just to get the basics of sie defined in UDB.
• dscratch0 and dscratch are generic registers which just contain a "value", so let's define a field, "VALUE" of MXLEN bits. See "mtval.yaml" for an example.

[FYI, I removed "ustatus, uie, utvec, uscratch, uepc, ucause, utval, uip, sedeleg, sideleg" from the PR description.]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.05%. Comparing base (15e139b) to head (132d6c8).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1236   +/-   ##
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  Coverage   46.05%   46.05%           
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  Files          11       11           
  Lines        4942     4942           
  Branches     1345     1345           
=======================================
  Hits         2276     2276           
  Misses       2666     2666           

Flag	Coverage Δ
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[ThinkOpenly]

Yay! CI passed!

Just a few nits, then we'll get this merged.

[ThinkOpenly]

You are encouraged to put your own copyright here, since this is your work! :-) Here, and the other files as well, of course.

(There are plenty of examples in the repo.)

[ThinkOpenly]

Remove all of the alias attributes. "alias" means that this field actually is another means of accessing the aliased field. Here, though, the field is standalone. It corresponds to the other register's fields for convenience, but is distinct.

[ThinkOpenly]

remove the * characters, here and elsewhere.

[ThinkOpenly]

If the field is definedBy the same extension as the CSR, then it does not need to be explicitly specified, so remove all definedBy: S attributes.