[llvm] [RISCV] Add scheduler model for sifive-p450. (PR #77989)

[Wang Pengcheng via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM.
Though I don't know the details about microarchitecture, I believe this scheduling model has been checked internally.

https://github.com/llvm/llvm-project/pull/77989