[mlir][nvgpu] Use TableGen TypeDef for NVGPU dialect types

Moves definition of DeviceAsyncToken to use the declarative Tablegen
TypeDef since the type is trivial. This also allows for removing the
current code for parsing/printing types by using the auto-generated
functions.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D134564