mmc: atmel: Add atmel sdhci support

The SDHCI is introduced by sama5d2, named as Secure Digital Multimedia
Card Controller(SDMMC). It supports the embedded MultiMedia Card (e.MMC)
Specification V4.41, the SD Memory Card Specification V3.0, and the SDIO
V3.0 specification. It is compliant with the SD Host Controller Standard
V3.0 specification.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>