mtd: omap2: allow bulding as a module

The omap2 nand device driver calls into the the elm code, which can
be a loadable module, and in that case it cannot be built-in itself.
I can see no reason why the omap2 driver cannot also be a module,
so let's make the option "tristate" in Kconfig to fix this allmodconfig
build error:

ERROR: "elm_config" [drivers/mtd/nand/omap2.ko] undefined!
ERROR: "elm_decode_bch_error_page" [drivers/mtd/nand/omap2.ko] undefined!

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: Afzal Mohammed <afzal@ti.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: linux-mtd@lists.infradead.org