Add an optional iDMA

[phsauter]

Generate a minimal configuration of the iDMA and add it to the repository.
Then integrate it into Croc while keeping the timing and area impact minimal, one option to avoid increased interconnect delays and size is to mux the ports together with the Debug and the cores D-fetch ports (as the debug port always has low throughput and the D-fetch has at most 1/3rd of the max throughput.
The default configurations remains croc without iDMA, it should only add one one/off parameter and the added complexity should be hidden as much as possible as to not increase the barrier of entry for the version without iDMA.