Transfer data to and from the MMC using DMA

- Add a method to transact with the MSDC interface using the DMA engine,
instead of using the FIFO directly.
- Fix some issues in the tuning where the return values weren't being
used -- this makes tuning 200MHz/8-bit happy
- Set the bus to 8-bit wide, and the bus-frequency to 188MHz (taken from
LK)

Change-Id: Ia3e8780093843a1f5892a8a89a974858355cccf0