Add MIPS extensions

[djtodoro]

Adding two extensions for MIPS p8700 CPU:

1. cmove (conditional move)
2. lsp (load/store pair)

The official product page here:
https://mips.com/products/hardware/p8700/

Upstreaming of this in LLVM is in progress:
llvm/llvm-project#121394

[djtodoro]

cc @asb :)

[lenary]

This looks good to me, and uncontroversial :)

[cmuellner]

Thanks for the PR!

I see that the references to the two new extensions go to the product description of the MIPS P8700 Series.
If that product retires, will the extensions also retire? If not (which I assume), is there a reference for a description of the extensions independent of the product they implement?

Also, the docs on the product page don't mention the two extensions (Xmipscmov and Xmipslsp). The appendix of the P8700 programmer's guide only lists nine custom instructions (ccmov, ehb, ihb, ldp, lwp, mtlbwr, pref, sdp, swp). This repo is not the right place to define extensions. Instead, a document should be referenced where the extensions are defined (incl. CSRs, instructions, etc).

A specification of a custom extension should also include the vendor prefix (e.g. mips.ccmov).