mtd: onenand: Rename omap2 driver to avoid a build warning

As previously reported by Sudip Mukherjee for the Samsung driver, the
omap2 onenand driver is called omap2.c in our directory and omap2.c in
the tty/serial/ directory. If both drivers are compiled as modules, it
would produce the following warning:

        warning: same module names found:
            drivers/tty/serial/omap2.ko
            drivers/mtd/nand/onenand/omap2.ko

Rename the onenand omap2 driver so that it fits the folder's
convention: onenand_omap2.c.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

drivers/mtd/nand/onenand/Makefile

@@ -8,7 +8,7 @@ obj-$(CONFIG_MTD_ONENAND)		+= onenand.o
 
 # Board specific.
 obj-$(CONFIG_MTD_ONENAND_GENERIC)	+= generic.o
-obj-$(CONFIG_MTD_ONENAND_OMAP2)		+= omap2.o
+obj-$(CONFIG_MTD_ONENAND_OMAP2)		+= onenand_omap2.o
 obj-$(CONFIG_MTD_ONENAND_SAMSUNG)       += onenand_samsung.o
 
 onenand-objs = onenand_base.o onenand_bbt.o