Update legalization of tf/tfl pad to the spec
need to work with a counterpart change in mlir: https://reviews.llvm.org/D143312 close this PR as discussed that the choice of using the operand type for those arguments has a better chance to support dynamic-shaped model.
Argument `padding` and `pad_const` are operands in the implementation, but in TOSA spec they are attributes.

Change-Id: I63046235597a2d0a6cc89c5178658b251862bd8b