[mlir][Linalg] Add a softmax op

This patch adds a softmax op.
For now, nothing interesting happens, we can only do a round trip.
Later patches will add the tiling interface and the lowering of this op to
a sequence of simpler ops.

This is graduating the linag_ext.softmax op from iree to LLVM.

Original implementation from Harsh Menon <harsh@nod-labs.com>
Nicolas Vasilache <nicolas.vasilache@gmail.com> co-authored this patch.

Differential Revision: https://reviews.llvm.org/D153422