mtd: nand: omap2: 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/mtd/nand/omap2_nand.ko | grep alias
alias:          platform:omap2-nand

After this patch:

$ modinfo drivers/mtd/nand/omap2_nand.ko | grep alias
alias:          platform:omap2-nand
alias:          of:N*T*Cti,omap2-nandC*
alias:          of:N*T*Cti,omap2-nand

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>