coreos-kernel: Adding MMC/SDHCI drivers

Adding the generic MMC driver as well as the corresponding SDCHI
components.  This will allow access for hardware which utilizes
MMC, SD, & SDHCI devices often used in embedded systems and for
emergency recovery partitions.
Merge pull request #908 from brianredbeard/mmc-sd-driver

coreos-kernel: Adding MMC/SDHCI drivers