mtd: dataflash: Export OF module alias information

The SPI core currently reports the MODALIAS uevent as "spi:<modalias>"
even for SPI devices that were registered by OF.

That means the OF module alias exported by MODULE_OF_TABLE(of,...) is
currently not used and user-space has no way to autoload this module.

But it is still a good practice to add the OF module alias information
into the kernel module even when it currently is unused so once the SPI
core is changed to report a correct OF modalias uevent, module
autoloading will be working for this driver.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>