[llvm] [RISCV] Move the RISCVII namespaced enums into RISCVVType namespace in RISCVTargetParser.h. NFC (PR #127585)

[Pengcheng Wang via llvm-commits]

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

Nice cleanup! Sometimes it is really hard to remember which namespace these functions/symbols are in.

https://github.com/llvm/llvm-project/pull/127585