[mlir][scf] Implement getSingle... of LoopLikeOpinterface for scf::ParallelOp (#68511)

This adds implementations for `getSingleIterationVar`,
`getSingleLowerBound`, `getSingleUpperBound`, `getSingleStep` of
`LoopLikeOpInterface` to `scf::ParallelOp`. Until now, the
implementations for these methods defaulted to returning `std::nullopt`,
even in the special case where the parallel Op only has one dimension.

Related: https://github.com/llvm/llvm-project/pull/67883