[llvm] [PowerPC] Update maintainer (PR #145519)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 approved this pull request.

Thanks for posting this PR. I left IBM last year and haven't worked on the PowerPC backend since then. We need new maintainers for PowerPC for sure. Let's merge this in first. And set the new maintainer(s) when there is any suggestion from @nemanjai  @lei137 @amy-kwan @diggerlin @RolandF77 or other PowerPC contributors.

https://github.com/llvm/llvm-project/pull/145519