mtd: nand: Allow MTD_NAND_OMAP2 to be usable on Keystone devices

Some Keystone devices (e.g. K2G) include a OMAP NAND IP.
Allow the NAND driver to be usable for both
Keystone and OMAP devices.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>