mmc: sdhci-of-at91: introduce driver for the Atmel SDMMC

Introduce driver for he Atmel SDMMC available on sama5d2. It is a sdhci
compliant controller.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>