mmc: tmio_mmc: allow multi-element scatter-gather lists

The driver is capable of handling multi-element sg lists in both PIO and
DMA modes. In DMA mode this also allows to use the DMA sg capability more
efficiently and almost doubles the throughput.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>