mlir/tosa: move tosa.pad from Linalg to Tensor conversion

Since tosa.pad is lowered strictly to artih and tensor ops, move
ConvertPad from TosaToLinalg to TosaToTensor, benefitting non-Linalg
Tosa targets. TensorToLinalg exists, and is trivial, so nothing is lost.

Signed-off-by: Ramkumar Ramachandra <r@artagnon.com>

Differential Revision: https://reviews.llvm.org/D139091