mmc: mediatek: 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/mtk-sd.ko | grep alias
$

After this patch:

$ modinfo drivers/mmc/host/mtk-sd.ko | grep alias
alias:          of:N*T*Cmediatek,mt8135-mmcC*
alias:          of:N*T*Cmediatek,mt8135-mmc

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>