[llvm] [mlir] [mlir][GPU] Move `GPUDeviceMappingAttr.td` to `GPU/IR`. (PR #95880)

[Han-Chung Wang via llvm-commits]

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

good catch! Yes, I think that the attribute def should be in `IR/`.

https://github.com/llvm/llvm-project/pull/95880