sd: Changes framework to support MMC

- Adds mmc.c
- Edits sd.c to init and probe MMC
- Adds mmc init to sd_init
- Some functions from sdmmc.c should be in sd_ops because
they can be used by both sdmmc and mmc.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>