mmc: sdhci-of-at91: Fix module autoload

If the driver is built as a module, autoload won't work because the module
alias information is not filled. So user-space can't match the registered
device with the corresponding module.

Export the module alias information using the MODULE_DEVICE_TABLE() macro.

Before this patch:

$ modinfo drivers/mmc/host/sdhci-of-at91.ko | grep alias
$

After this patch:

$ modinfo drivers/mmc/host/sdhci-of-at91.ko | grep alias
alias:          of:N*T*Catmel,sama5d2-sdhciC*
alias:          of:N*T*Catmel,sama5d2-sdhci

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>