[clang] [RISCV] Mark the Zfa extension as non-experimental (PR #68113)

[Alex Bradbury via cfe-commits]

asb wrote:

> Please update the header comment at the top of RISCVInstrInfoZfa.td

Thanks, split that change between #67964 and this one.

https://github.com/llvm/llvm-project/pull/68113