mmc: sh_mmcif: simplify platform data

Provide platforms with a simplified way to specify MMCIF DMA slave IDs in
a way, similar to SDHI and other sh_dma clients.

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