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>