[llvm] [RISCV] Fix Enum for Custom Vendor CSR encodings (PR #125172)

[Sudharsan Veeravalli via llvm-commits]

svs-quic wrote:

There are no tests for this right now since there aren't any vendor CSRs in the codebase.  I have pushed this in preparation for upstreaming the Qualcomm Xqci custom CSRs.

https://github.com/llvm/llvm-project/pull/125172