mmc: sdio: Add helper macro for sdio_driver boilerplate

This patch introduces the module_sdio_driver macro which is a convenience
macro for SDIO driver modules similar to module_usb_driver. It is intended
to be used by drivers which init/exit section does nothing but register/
unregister the SDIO driver. By using this macro it is possible to eliminate
a few lines of boilerplate code per SDIO driver.

Suggested-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>