DMA controller

[jbush001]

Currently the VGA controller has a bus master to read video data. The interconnect only has two ports (one for the processor and one for VGA), which precludes adding other peripherals that support bus mastering/DMA. Move code from VGA into a common DMA controller with multiple channels and have VGA use that. Use remaining channels for other peripherals.

(this is not part of the Nyuzi core proper and is an FPGA test harness improvement)