SGDMA debug

Someone other than me :) Altera will support the mSGDMA moving forward as an embedded IP just like the rest of the soft IP cores that are available in Qsys. I think the right approach is to have a frontend unit become part of the mSGDMA instead of trying to make the dispatcher handle this since the dispatcher is really designed to just execute descriptors in the order that it sees them. With a frontend that handles linked-lists then this becomes a more robust solution. This will also give similar functionality to the SGDMA that has been in Qsys for years except it's just an optional feature for those who want linked-list descriptor prefetching.