[mlir][AMDGPU] Define amdgpu.mfma operator

The amdgpu.mfma operator is a wrapper around the Matrix Fused Multiply
Add (MFMA) instructions on some AMD GPUs (the CDNA-based MI-* cards).

This interface allows for selecting the operation to be performed by
specifying the dimensions of the multiplication to be performed and
any additional attributes (such as whether to use reduced-precision
floating-point math) that are needed to select the relevant mfma
instruction and set its parameters.

Reviewed By: ThomasRaoux, nirvedhmeshram

Differential Revision: https://reviews.llvm.org/D132956