mmc: davinci_mmc: set MODULE_ALIAS to allow autoloading

Davinci MMC platform devices (as in mach-davinci/devices-da8xx.c) use
"davinci_mmc" as their name. To allow autoloading of the relevant
driver, the module needs to set the MODULE_ALIAS.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>